hello i'm claire reimers president-elect of the ocean sciences section of agu this year's william and carolyn rieberg lecture will be given by dr dennis hansel in recognition of his significant contributions to the fields of global biogeochemistry and marine geochemistry dr hansel is a professor in the department of ocean sciences at the rosensteil school of marine and atmospheric science at the university of miami where he's worked since 2001. his research focuses on the role that dissolved organic matter or dom plays in the carbon and nitrogen cycles in the ocean and he is highly respected for producing a wealth of measurements that reveal the biogeochemical processes that control dom dynamics dr hansel earned his phd at the university of alaska in fairbanks working with john goring on the nitrogen cycle in the highly productive bearing in chukchi seas interestingly bill rieberg served on his ph.d committee and so contributed to dr hansel's dedication to making high quality biogeochemical measurements having spent more than two years at sea of war research vessels dr hansel's research has taken him to all the major ocean basins and to all the continents this research has resulted in over a hundred academic articles in journals such as science nature oceanography and global biogeochemical cycles and it's led to two editions of the highly regarded reference book biogeochemistry of marine dissolved organic matter which he edited with craig carlson dr hansel has been honored in the past by giving agu's harold sphere to fletcher in 2004 he was named an agu fellow in 2019 and he became a triple a s fellow in 2018. in terms of service to the community he's chaired the united states carbon cycle steering group he's a trustee for the bermuda institute of ocean sciences and he's the incoming chair of the unil's council which is the governing body of the u.s academic research fleet during this time of covet dennis reports that he and his wife paula have taken advantage of being homebodies but that they take long bike rides each morning even through the worst of miami's summer his lecture today is entitled oceanic dissolved organic carbon the world tour and there will be a live question and answer section session immediately after the lecture thank you thank you claire for that wonderful introduction it is a very special honor to be able to speak to everybody uh now this evening uh for the uh bill rieberg and carolyn reberg lecture you know obviously there's a photo of carolyn and bill and i had the great pleasure of meeting carolyn back in the mid-1980s when i was a student at the university alaska fairbanks and made it up to the rieberg house a few times for celebrations when bill's students would graduate and it was always such a fun time and such wonderful people and so bill has been a good friend and important mentor to me since early in my career because again i was a graduate student where he was on the faculty and he was a member of my committee so again it's a very special honor for me to be able to give the river lecture today thank you for joining me so i want to start with some context you know what is we'll be talking about saltorganic carbon in the ocean but there is an analogous large reservoir of carbon in the earth system and that is soil organic carbon both soil organic carbon and the oceans dissolved organic carbon are sequestration products of the earth system you know so the net community production or the net ecosystem production of the system generates these products that have long lives and in the terrestrial environment that material is organic matter that accumulates in the upper soil and soil organic carbon and there's a lot of it so uh this photo at the top here has uh a forest out of sitka alaska some needles falling down into the soils and whatever isn't consumed by the heterotrophs will accumulate in the soils so that especially in the high latitudes of north of the northern hemisphere there's a lot of organic carbon residing in those very cold and particularly the frozen soils of permafrost soils the soil organic carbon has on the order of uh 2000 petagrams of carbon and a pentagram of carbon is a billion metric tons so 2 000 billion metric tons is a lot of carbon so in the ocean it's not trees it's phytoplankton they're fixing co2 and they end up with a byproduct of of their growth that goes into solution in the ocean and it's dissolved it's dissolved organic carbon it's a very complex material as far as composition but it is being spread out very dilute uh dilute nature of it in the ocean because you know say it's 5 000 meters of ocean global ocean a lot of volume so there's 660 pedograms of carbon there are 660 billion metric tons and you can see in the soil most of that carbon is in the upper meters of the soil in the ocean it's distributed throughout the depths and at much lower concentrations than in the soil so the soil organic carbon you know that community that's been studying that carbon knows that it's locked away especially in frozen soils they know that as that soil warms and thaws then the microbes will be able to uh metabolize the organic matter get energy from it and they're going to release that carbon as co2 to the atmosphere so they have a mechanism you know thawing the soil and the sensitivity it's going to respond to warming of the atmosphere in the ocean we we don't know yet enough about it to say that we can predict uh that there that it will have a sensitivity to anything we just don't know the fundamental nature of that pool and that fundamental nature is uh what i've been working on and trying to understand for the years that i've been focused on dissolved organic carbon in the ocean so we're going to take a look for the first three slides just a global kind of view it dissolved organic carbon in the ocean just to give you a sense you know more context essentially and uh this this is a plot showing uh observations these are all of observation data uh from three sections the atlantic over here the pacific over here indian ocean over here and you can see there's a change in the depth scale and what we see here is that we have enrichment at the surface and we have lower concentrations of depth and throughout this talk i'm not going to talk to you about concentrations per se i'm not going to say it's 50 here and it's 60 there i'm just going to refer to colors so that you can know that if you see blues and pinks it's low if you see greens it's intermediate if it's red it's high all right that's basically all you need to know we don't need to know the absolute values if you do need to know you know i will always have the citation there that you can go look these things up so we have this large pool 660 pedograms and most of that is 640 pedograms of carbon and that is the blues and pinks in the ocean so this is the background concentrations they're kind of low concentrations just to put a number out there i just said i wouldn't do it but about 40 micromolar in the deep ocean more or less and that's the carbon that has the uh the great radiocarbon age associated with it so it could be 5 000 years in north atlantic it could be 6 000 years in the north pacific so that means that that carbon in a bulk sense is just circulating with the overturning uh the overturning limbs of the ocean and uh has a long life longer than the circulation time of the ocean and on top of that up in the euphodic zone this is where the organic matter is being produced by the autotrophs you know primarily fundamentally heterotrophs have have a contribution to make but there's about 20 pedograms of material there it's in the reds and the greens and the oranges so it's it's it's accumulated on top of those blues and pinks think of it that way and so all of that carbon that 20 petagrams of carbon is modern carbon and then the 640 is older carbon some thousands of years of age and uh these these black arrows just tell us what the circulation and some water masses and the white lines are telling us some uh isopickals but we don't need to look at those right now so here is a distribution in the upper ocean this is a combination of modeled results so when you see the uniform color in the background that is a model result and when you see the little circles with the colors in them that those are observations so the observations are done on these long lines uh currently it is the u.s go ship program and craig carlson and i and craig is at uc santa barbara have been making these measurements for a long time and i think my first long line measurement was this line over here p18 in about 1994. and we've been doing it ever since and craig has been a fantastic colleague and friend and because of that you know strong relationship we have in our work it's you know really made made progress for me in understanding the system so i appreciate all of his contributions so when i look at this i'm seeing uh distributions that are interesting i see enrichment in the low and mid latitudes i see intermediate values say in the far north atlantic there and then i see the lowest concentrations in the southern ocean and you know first pass we'd like to understand why is that distribution the way it is and uh there's an important role for the main thermocline here you know where we have the highest concentrations of doc we also have the warmest waters and the warmest waters exist at those low and mid latitudes not just because the sun is being on the upper ocean there but also because there's a stability in the upper water column that allows that heat to be retained it isn't going to be easily mixed out to the deep ocean so it's going to that's going to have a role as well for doc distributions so here's our main thermocline just a little schematic of it and it runs from let's just say 40 north to 40 south or 45 45 something like that and uh it's it's providing stability and and retention of the upper water column and giving some residence time to that upper water column and so there's the approximate domain of the main thermocline all right so i'm running north to south and and offering that stability and so that's where our highest doc concentrations are so even if doc accumulation there is slow the residence time of the water is long enough that doc can actually accumulate and reach the highest concentrations in these kind of environments now once you have accumulated some doc now it's going to be transported advected for example in north atlantic it gets away from the stability of the main thermocline and so this little plot down here shows it turning green and that just means that we're you know we're starting we've lowered the concentration because we've got some additional vertical mixing going on because we don't have that enforced stability or lack of mixing and force by the main thermocline and then it goes continues up with the overturned circulation and then there's some consumption of that organic carbon so that you come back with some lower concentrations back to the 40s you know 40 micromolar so and then that deep water is pulled back up to the surface uh southern ocean in the divergent zone and that's what gives us this blue and pink distribution these are the waters that have been returned being returned from the deep waters of the atlantic the pacific and indian oceans and they're all relatively impoverished of dissolved again at carbon hence that blue and pink distribution again this is enriched because in large part because of the stability imparted by the main thermocline okay so we have a sense for what controls that uh surface distribution and here we have that same plot up here that water's being brought up and as we said it's going to be caught up in the overturning circulation and these again are model distributions because we don't have the didn't put the observations on top of these and so now we have at 3 000 meters here we have enriched doc in the far north atlantic works its way down in the deep western boundary current northland deep water it's caught up in the circumpolar circulation and then some of that water is going to be working its way up into the far north pacific and so what we see here is then the highest concentrations where we're introducing those doc enriched surface waters to depth and that's in the north atlantic and we see a big concentration gradient from the north atlantic to the north pacific so what we'd like to do in this talk is to understand more about the dynamics of the doc the surface ocean and not just that it's kept up there by a physical feature but what about the biological nature of its production you know can we quantify the production in the upper ocean you know where is it taking place how much is taking place and that sort of thing and then we'll come down to the subsurface ocean and i'd really like to understand the gradients that we see in the deep ocean so those will be the two parts of the talk that are coming here all right so this is an important message at the top there doc is a product of the neck community production of the system we saw that with soil organic matter or we saw that you know i introduced that concept early on anything that escapes consumption during you know a productive season and you know it's it's not consumed by a heterotroph that's a net production of the system and doc the fact that it's accumulating then is a product of that net community production and so that's that's a relationship that is important for us to understand because we can get to net community production and from that if we understand the relationship with the oc then maybe we can project what the doc variability is based on our observations of net community production so and here's a note down here necron production can be observed as the net biological drawdown of co2 in the ocean right so if something is the autotrophs are fixing co2 and making biomass or making solve organic carbon if it isn't mineralized by a heterotroph there's a net drawdown of co2 and that's our measure of net community production or we can use new nitrogen consumption so we don't always have co2 measurements such as nitrate and euphotic zone alternatively you can look at the net organic matter production itself look at the products of net commute production so it helps it's helpful to look at the time series how do things change with time we can look at net community production change with time we can look at dlc change with time and that's where we really uh started learning about this relationship and and that's at the bat site in the northern sargasso sea the bermuda atlantic time series studies site run out of the bermuda institute of ocean sciences i'm going to show data there that craig and i worked on early and then i'm going to jump over to the gulf of alaska and see if we can extend our understanding from what we learned in the sargasso sea and then i'm going to jump back to the entire atlantic basin and see if we can again extend our understanding of the system all right so here are data these are climatological data from the bermuda atlantic time series study site or bats and what we're going to learn here or see here is that doc accumulates seasonally as a product of the net community production so let's take a look at the system here winter values all right so the top is uh this is february into early april top is temperature so it's the coldest time of the year out there in the northern sargasso sea and and looking at those temperatures it's not all that cold but it's the coldest time of year and then that's when the chlorophyll a concentrations are at the highest you know they're reaching you know elevated values up to the surface and that's because that's when the nutrients are being mixed up with convex convective overturn now the next plot down is tco2 and that's being mixed up from below during convective overturn as well so the enriched values down here say 150 200 meters are being mixed up to the surface and then the dissolve the anti-carbon distribution that's when we have the lowest concentrations because we're taking up this low doc deeper water and mixing it up towards the surface so it dilutes what was up here and so it lowers the concentration now let's look at summer condition nice and warm very highly stratified seasonally stratified chlorophyll is limited to the deep chlorophyll maximum and tco2 now is quite low relative to what it was there's about a 50 micromolar drop from this period in the winter to this period in the summer and dissolved organic carbon has increased so it's gone from these greens into these reds here so what we're seeing this is the thing that we're after is to to know that the tco2 drawdown that is biological you know co2 is being lost with warming of that water and by degas in the atmosphere but there's a biological component so of the tco2 pulled down or removed 30 or 40 percent is biological and then we see associated with that that's the measure of net community production then the tco due to drawdown by biology and then associated that with that is this accumulation of doc which is all biological so what we learned when we started doing this kind of work is that we need to establish that relationship between the amount of organic carbon that accumulates doc is a product of the neck community production if we know how much nectar production is then we need to figure out how much of that is accumulating this doc and then we can and then we can make some headway with that knowledge let's go over to the gulf of alaska and and this the data show is really focused on ocean station papa where that star is the red line is uh line p occupied by the canadian scientists doing great time series there they occupy it three times a year and we've been involved with some of that work as well so the message here is that if we can get the net community production if we can estimate net community production and if we know how much doc accumulates as a function of that connect community production then we've got uh predictive capability and that's what we're after here so let's take a look at what the data would look like out in this system now at ocean station papa we don't have a lot of inorganic carbon measurements but we do have a lot of nitrate measurements and nitrate is going to going to be our our value that we use to get at the net community production so you can see here that nitrate concentrations in january winter time or some value and then they decrease into august so that is our measure of the nectar production in the system in nitrogen space and we would convert that using molar expected molar relationships to carbon right so we can get ncp to carbon with this knowledge and then below we see wintertime doc as at the bat site its lowest in the winter time and then it can accumulate into the summer so that's one of our products the net community production product it's that relationship between how much is accumulating relative to the ncp that we really need to establish and in this system that relationship suggests that it's about a quarter of the ncp accumulates as doc in the euphodic zone so that's an important thing to know because if we can get ncp then we can project what the doc variability must be but how do we get a lot of data on ncp out in that system if we can get a lot of nitrate data and the seasonality of it then we're then we can get to ncp and so we can do that using these by geochemical argo floats and so this is just a a fargo float mission you know it's at the surface works its way down to in this case 2000 meters and then it ascends to the surface and while it's going up it's acquiring data for whichever sensors it has but it's going to be salinity temperature pressure and then the biogeochemical variables and the most important one for us in this situation is nitrate and so these floats are out you know in certain areas they're going to be highly populated once the uh mbari and the university of washington and some other institute scripps and princeton are able to populate the ocean with uh many more of these floats these go argo floats uh but we we have some historical data for 10 years of data we've had these floats and the black lines indicate their path just working their way around ocean station papa and so we can use those nitrate data and that's what we do here so it's 10 years worth of data you can see the seasonality right so the winter cool periods and the summer warm periods the colors are all temperature the lines are all the iso pigments and below that is nitrate so these these argofloats had nitrate sensors and so we see uh summertime lows in nitrate and wintertime highs in nitrate so the nitrates mixed up in the winter and then it's consumed and that consumption that delta nitrate gets to our net community production in the euphotic zone uh right we've got it as a consumption of nitrate we can use a molar red field type ratios to get us to carbon in ncp so that's what we do so now we have uh ncp 10 years of ncp data and we know that the relationship was 26 of ncp was accumulating as doc right and so we can plot what the doc variability would look like even though that we're not out there to measure it for those 10 years but this is what we project you know the system the variability in the system to have uh you know what what we expect to have occurred out there in that system because this 26 uh is uh you know for all the data that we observations to establish it you know it's a pretty pretty robust number and even if you change it you know a few percent either way it doesn't really change this distribution very much as it turns out so you know what we see here is that we had a couple of particularly warm years this was uh the warm blob year that was the el nino year that followed it dramatic impacts on nitrate these two years here with some consequences for the doc distribution as well so uh you know it was very nice to be able to get a sense for what doc is doing out there just based on what we the data we can get from the argo floats all right let's go to the atlantic so we think then that we have some sense for you know the best way to get at doc variability without directly measuring it because we can't go around everywhere measuring doc temporally you know look at the time series everywhere but we can often get uh ncp net commute production because we'll have more data especially for nitrate especially as those biogeochemical argofloats come online in increasing numbers so the rule that we've established basically is that the amount of ncp controls seasonal dsdoc accumulation and the message then is we should be able to predict those doc distributions once we know how much doc accumulates in the system relative to the ncp so that's that's what we're going to try to do in the atlantic here but we're going to be looking at it in a very static sense we're not looking at it as a time series we're just looking at the distribution so these are the observed distributions and you can see the zonality all the oranges through here the equatorial upwelling causes the greens through here the orange is through here the greens through here the pinks and the blues through here etc etc so a lot of zonality in that distribution and the question is based on what we know about the role of ncp and how much of that accumulates as doc can we reproduce that distribution just based on those ncp values for the system and and again we have to have that delta doc how much dlc accumulates as a function of net production and in this system yeah i i just you know suspect that each subsystem has its own value for the amount of doc accumulation is ncp so an upwind system probably a little bit different than a subtropical system but a sensitivity analysis for the entire atlantic for all of this area that we're looking at gave a value that you know had the best returns for the whole basin at about 17 so that's the value that we use right so we have to get ncp how do we get ncp this is the simplest one of the simplest things that i've ever done to get to ncp and it it's surprising that it actually worked so we can roughly characterize ncp by the change in nitrate with depth in divergent zones so uh you know in the summertime nitrate is low throughout most of the surface atlantic it's high in the divergence zone of the southern ocean high in the divergent zone of the sub subarctic upwelling system here and then at 100 meters just picking a depth to say you know where is there a lot of nitrate so we see it there in that divergent zone we see these are all the divergent zones that we see in this divergence zone this one and this one so i'm reminded of the of the models you know that people used to run to to estimate how much net production there was or how much new production or how much export production and those were uh nutrient restoring uh models basically so you know that we knew that there'd be very little nitrate at the surface we knew how much water was being lifted up the you know the physical nature of the system was reproducing the model and so then we'd restore you know the modelers would restore to these low nutrient concentrations and that delta and the nutrient from what had been upwelled to what has observed the surface was your measure of how much new production was in the system so we did the same thing as a very but a very static look we just take some nitrate it you know subsurface nitrate compare it to the distribution at the surface make that our net community production that delta and then apply our ratio of how much of that accumulated as doc and this is what we came up with okay so this is doc estimated simply from the change in nitrate concentrations in the vertical and uh all the lines are again the observations and the background field is simply taking that that ncp from the static look of the ocean and it worked pretty darn well you know just surprisingly well so where it's green it's green where there's orange there's orange a little bit of an upwelling signal here you know we saw green it comes off as orange in the calculation greens blues and pinks but we don't see you know the the little model we did doesn't capture this material so this is this model though is without any circulation right it's just a static look nitrate it subsurface bring it up to the surface and and restore that surface back to low concentrations so if we then turn on some circulation we bring the carbon that's formed over in the upwelling systems and the diversion zones across across the atlantic zonally and that we can populate the caribbean for example with these higher concentrations of doc so this again surprisingly works surprisingly well you know the distributions where it's green is green blue and pink or where they should be there's this interesting little location here every time we occupy this area we find elevated doc we don't know what the what the mechanism is for that what's the underlying cause of that all right so we i think we have a pretty good sense that if you can get the net community production if you have in a sense for if you have a sense for how much dfc accumulates as a function of it then you can do a pretty good job of projecting what the distribution should be in the ocean and the surface ocean what the what the temporal variability should be in the uh surface ocean and so that's where we are with that it's you know nice progress i think so these surface waters then in the north atlantic are going to be carried up to the far north there we saw that in the introduction and it's going to be exported with overturning circulation so let's start focusing on the deep ocean now so these are observations these are three the three lines uh in the north atlantic that we typically occupy these meridianal sections and you can see the reds at the surface that's all that surface accumulated organic matter right the the autotrophs up there producing those dissolving added carbon such that it accumulates up there and recall now that we take some of that material and bring it up to the far north for overturning circulation which we're then are going to going to see in the subsurface layers so here's here's our little circulation for northland deep water following as a deep that's the deep western boundary current and so we can see the enhanced doc concentrations on the northern part of this line the northern part of this line the northern part of that line and then the southern parts of these lines because this enriched doc is being brought down through that system there so that's really a beautiful thing to see uh it shows us that the system works exactly as we expect but it's nice to have the data it makes it a beautiful uh a beautiful system all right so let's let's see what happens at the south atlantic so let's carry this line further south into the south atlantic and here we are we've looked at this right we've seen this deep western boundary current and then it comes off into the south atlantic and we can still see that beautiful enrichment of dissolved organic carbon in the heart of the north atlantic deep water and we can see that northland deep water and this doc that's in it being pulled up into the divergence zone of the southern ocean there so uh it's it's doing what we expect it's nice to be able to see it though it gives us a sense of the lifetime of this material you know how far is it transported over what time frames that sort of thing so again this is this is dlc at 3000 meters deep right so the model has you know the enrichment at the surface bringing it down here and then you can see the greens down in this area and the model must be thinking well i got some north length deep water down here because that's what this carrying this so i'm going i'm going to have some extra doc in this section as well so let's take a look and see if we can find that dissolved organic carbon so we have a section here that we have data from that will show the features very nicely in the upper plot again we're right on the section 90 degrees east is salinity upper plot of salinity and the big orange feature is northland deep water you know beautiful salinas beautiful tracer of north length deep water and then the below it is the lower circumpolar deep water and just above it is indian ocean deep water and then the low salinity antarctic intermediate water so we can see that location of that northland deep water very well and then the lower plot we have the delta c14 of the doc and uh the scale over here you know the more negative the value the older the carbon in the bulk doc pool and uh the the less negative the value uh the more modern component in that pool and so the observations are in the circles and the the statistical treatment the multiple linear multiple linear regression analysis to extend these observations into the field is is in the color background here and so what we see very nicely is right in the heart of this north atlantic deep water we have these uh more modern younger components of dissolved organic carbon so that's a that's a beautiful thing to see because you know we've taken surface water for the north atlantic which has a strong modern carbon component to it we've brought it up into the you know greenland norwegian sea area and overturned and then sent it south and it's still carrying that modern component and we bring it over here into the indian ocean and here we see that signal still all right so so just to the same extent that northland deep water retains its signature with its with the salt field it's retaining a signature and in the uh radio carbon content dissolved again at carbon pool as well so that very nice to see that sort of a feature all right so here we are that same image we're taking that enriched doc down gets caught up in the circumpolar waters we saw some up in the in the indian ocean and then some of it's going to come up as bottom waters and work its way up into the far north pacific so when we look at this distribution you know i was trained originally as a biologist and so when i see organic matter decreasing essentially with time then i'm going to think that biology has an important role there and certainly it does in places especially in the far north atlantic i suspect it has an important role in reducing some concentrations there but while we see big gradients in the ocean we also have to wonder about mixing mixing plays a role in in so many gradients that we see in the ocean we have to wonder about the gradients in doc it particularly when we look at the distribution of salinity at the same depth as for doc we see the same gradient right we see high doc where we see high solidity we see low salinity and we see low doc so we know this is conserved right so the high high salinity is coming in with that overturning circulation so we we really need to understand the gradients in the salinity to understand the gradients in doc is it mixing is there's a biological component is there some removal or addition going on so that's what we're going to do next so we need to understand uh salinity at great depths in the pacific to answer the question so this is salinity uh from the long lines from the repeat from the waste lines and and the us go ship and that sort of thing uh salinity at 4000 meters and so we can see the high salinity waters some north length deep water in fluids here working its way far into the north uh deep north pacific but we also see a strong gradient a very very obvious reduction in salinity up in these uh far northern latitudes so we need to understand that and then come back to look at doc distributions in that system so here's a little schematic you know this is how i think about the system so here on the south pacific and this is along this line that we're running the schematic south north high salinity coming in at the bottom right right in there so there it is the slower circumpolar deep water working its way along the bottom now we have to add freshness and freshness you know there isn't uh ventilation of the deep ocean from the surface in the north pacific and it's a closed basin so we're not transporting things in from the side so the freshness the lowering of the salinity has to be associated with this these intermediate waters that they're very low salinity you know above these deep waters all right so you add some freshness add some warming from geothermal sources and change the buoyancy of the water it rises to two to three thousand meters or so and then it returns back to the south as pacific deep water or north pacific deep water continues to the south and it too as i said earlier is going to be pulled up into the divergent zone of the southern ocean so this is the fundamental uh message for establishing this gradient salinity we have to bring in some freshness and so we're going to look at this system we're going to consider this an end member and we're going to look at this as an in-memory and in salinity space everything that we see in this gradient is established by mixing right so we use that to our advantage so these are the data from that same section all right circumpolar deep water north pacific deep water and returning to the south and so here are in members so the southern ocean high salinity and member here's the intermediate water this low salinity in number and it's mixing between these in members that creates the entire gradient for salinity in the deep pacific here here is dissolved again a carbon observed in the system and we have the same n members right there's there are in members southern ocean and more of the intermediate waters and what's different here is that the in-member concentrations of dissolved organic carbon are both higher than all of the doc concentrations here in blue all of these blues are lower concentration than the greens and the oranges so that tells us that there is removal of organic carbon going on in the system it's not conserved so let's see if we can uh take a closer look at that here's the same salinity distribution that we just looked at here's the same distribution of observed doc because we have we can identify n members and and we know what should be there in doc space if if it's conservative we have salinity as our tracer we can compare what should be there versus what is there to tell us what is missing from the system and that's what the lower plot is this is the doc that is missing from a conserved world in the deep pacific ocean as far as doc and so what do we see down here we see a big removal you know it's on the order of four or five micromolar being removed in the in the deep north pacific here on the order of 1000 meters 2000 meters 3000 meters and then we see another similar feature here in the south pacific so these are these are doc sinks that are are that existed out there and they i refer to them as local doc sinks they're local because they're happening in these specific locations they're not happening everywhere right we can go out and see these you know that there's a removal going on there but when we look at another part of the system say along the bottom there all this doc enriched water work its way along the bottom to the north this carbon is all conserved all right so that that is a very interesting feature the doc is conserved it works its way up to some depth and there is a removal transported to the south pacific and then there's further removal down there by what process we don't know abiotic biotic a lot of uh thought needs to go into that i hope some someone will be thinking about that all right so i told you this was conservative transport let's let's see if it is let me prove that it is so we can do a mixing experiment here we're going to mix uh the southern uh in-member with this northern end member and that's going to be our low doc water as an end member all right so we're changing the northern end number to the low doc because the gradient we have is from high to low so we need a low doc member to mix with this to see how conservative this is along the bottom so the same data just a little bit different look in the plot salinity enriched here selenium rich there low salinity here do you see this observation do you see a rich deer doc enriched here low doc here it's starting to look like it might be conservative but let's just test it we'll use these in-members you know as our conservative tracer we'll use these in-members to tell us what the doc field would look like if it was conserved and just mix those and this is the doc distribution in a conserved world it looks exactly like salinity because it is salinity it's just converted to doc when you do these mixing experiments but you can see it's you know very much like the observations you know the observations nature's involved right so there's some variability but basically we have enriched doc in the southern ocean we have rich enriched doc at the bottom of the equator and we have low doc at the bottom in the north and those are all what we expect based on conservation so it tells us that this doc near the near the bottom of the equator for example is being transported in with this higher salinity water which ultimately would have come from the southern ocean and you know work its way to the equator and then along the equator possibly so that that's an advected feature there so what we see here then is the calcium dosa when we're looking at these deep waters this is really biologically recalculated material it's largely conserved during this transport once it gets up here there's a little bit of a sink and once it gets up here there's a little bit of a sink so we don't know what the mechanisms are for that yet okay so i've been telling you that things look pretty you know you get some removal going on in some places it's pretty conservative in other places but there's a lot of dynamics going on in the deep ocean with doc that are are are on top of those conserved distributions but they they're going on so we need to take a look at that as well so on top of that conservative behavior we have particles sinking down and they're solubilizing and leaving the oc behind and that's what supports the deep microbes right the prokaryotes in the deep ocean so this plot is uh prokaryote abundance by toshi nagata in the pacific and here we are in the southern ocean divergence zone there's some export going on so we have enriched prokaryote abundance and the deep waters there the equator divergence zone a lot of x relatively high export going on enriched prokaryotes in that system and then we have divergence in the far north pacific export and enrichment the prokaryotes in the sub under the underlying subtropical systems a lot less export that which is exported isn't going to you know get down very deep and so the microbial numbers are much lower so so i show this not only because it's a beautiful distribution but because it tells us that it tells us where those particles are the export is important where solubility solubilization of doc is important as evidenced by prokaryote abundance all right so we know particles think it and they're solubilizing so we have some data that show that sort of thing here are some data from monica orianna paper we did in 2012 and these are rabisco distributions so rubisco is the most is said to be the most abundant enzyme on the earth and it's the first enzyme for fixing co2 by autotrophs so if you have an autotroph anywhere it's going to have rubisco fixing that co2 to to make more organic matter and so hence it's hence its great abundance on the earth and as it turns out those sinking particles are going to be containing autotrophs therefore they're going to be contain containing rubisco and so we see this beautiful column of enriched rubisco concentrations here and this is you know we're pretty far north we're up in the sub-arctic system there and then we on the equator you know this column of enrichment of rubisco here and then while we're in the subtropical waters we'll get the blues right lower concentration because of lower export so this is a very nice tracer then of those particles sinking down and solubilizing and the interesting thing this is inside of this rubisco it seems to have some life to it so we have silicic acid concentrations in the lines here and you can see where they're bowing to the south that's that pacific deep water driving isolatic acid waters to the south and it's pulling rubisco and rich waters to the south as well so that's a that's a very nice transport going on a nice feature to be able to see okay so we're going to take a look again there's that overturned circulation we're going to take a look let's see if let's go back and see if we can find doc elevated in association with this rubisco so here's that section we've been looking at in the middle of the pacific and uh you know here's our sinks here and here and our conserved transport along the bottom there and this section was done uh this is late 2005 november december time frame and then this is uh more or less closer to spring uh march up here in the far north of 2006. so let's blow this area up take a closer look at it and there we go this is north pacific doc in march 2006 and we do see these enrichments these columns of enriched doc and this column in particular is exactly where that rubisco column was present so that's a nice correlation and we wouldn't have vertical columns of doc without vertical inputs because of the overturning circulation that we've been demonstrating here so that was that's a nice correspondence to observe so this is late winter march 2006 late winter what's it look like later in the year maybe during the spring bloom periods or when there's a lot of export let's take revisit the same location in june 2015 much more dissolved organic carbon in the system uh we still have these columns showing up they're not always in the same location you know this is going to be associated with fronts and things like you know that focus export and this column in particular is interesting because uh andrew mcdonald university alaska fairbanks measured uh optically measured particle abundances and these are large particles greater than two millimeter through the water column and we see a very nice correlation between particles in the water column and uh this enriched doc column as well just as interestingly uh we have an enriched you know a column of enriched doc without particles now this location would have experienced uh exports where the particles would have been in the water column a month or two before the crews before the ship came along so when we were out there this was going on but a month or two earlier export would have been taking place down in this location and the particles have left the system but it appears to have left a column of residual doc there to support those microbes yet i put that yet there because all of this dynamic is going on we're adding obviously adding doc to great depth with the sinking particles yet we could do that analysis earlier where we saw a sink here we saw a sink there and we saw a very conservative transport here so all of this conservative look and including the sinks that's that recalcitrant pool that you know has some conservative behavior all of this material that's being added on top this is that modern carbon so these are uh organic matter uh with the composition that is going to be pretty recognizable by deep microbes proteins lipids carbohydrates it's going to be what biomass is made out of it's going to sink down there and so i think the microbes can do a great job pulling that down and and in the background you have a much different dynamic for that uh great recalcitrant pool of carbon all right so let me summarize here where what we've taken a look at today large scale dynamics as i as the title said it's a world tour uh it's fun to fund a tour of the world and uh we didn't get to do the arctic ocean i'll try to get to that a little bit in the question answer period so what do we have we have local input the north atlantic is where we're feeding the deep ocean with waters that are enriched dissolving into carbon we don't see that in the southern ocean the bottom waters do not enrich with doc it requires these waters that are subtropical that are enriched in dfc to work their way up to high latitude and be involved with the overturned circulation in the southern ocean the very strong polar fronts prevent that transport of this doc to contribute to bottom water formation so the bottom waters are not particularly you know not observably enriching the deep ocean like the north atlantic so the north atlantic really drives uh the uh input to the deep waters and that's obvious over here in the uh at the deeps you know 3000 meters or so so it's it's arriving at depth here and then continuing down to the south we saw that in with the northland deep water works its way around the circumpolar waters and up to the north pacific but but we have a gradient and so the only way the way we establish the gradient is to create sinks we have a high-end member now and then we have low end members where these sinks take place and one of those things chiara santinelli's written a paper suggesting that the deep mediterranean exhibits the properties of doc sinks so that's that that becomes the third one and you know we can't i haven't observed others in the global ocean so we have one high-end member we have some multiple low end members and then there's a mixing going on right so you have to mix between a low end member and a high in number in this drawing it's just between the southern ocean and the far north pacific and that's conserved and it's not particular it's particularly surprising that there's you know apparent conservation because again this carbon has a much greater age than the circulation time of the ocean so it you know there's going to be a conservative element to its transport and its mixing mechanisms and their sensitivities we don't know how things work right that's the mechanistic side if you don't understand how that works then you really can't assess sensitivities so if we go back to the soil organic carbon remember that to mobilize that carbon with you know the climate starts warming the local air warms you thaw the permafrost that makes that organic carbon available to the microbes they break it down they respire it co2 comes back out there's mechanisms and there's sensitivities all sensitive to the uh the temperature we don't have that yet yeah for dissolved organic carbon in the ocean we're still at the point of just establishing global distributions establishing temporal variability and fundamental controls on that variability so that's where we are at and we really need to make the next step to understand those mechanisms in order to get the sensitivities and one way one reason i i emphasize this is because paleo oceanographers have told us that the the the global ocean must have had much much more dissolved organic carbon uh in the past than it does today because uh you know they they some people employ it as a source of co2 to the atmosphere that would drive hyperthermal events 48 million years ago 50 million years ago so they're looking for a pool of carbon they can mobilize and the deep ocean full of doc is it but it requires much more doc than is observed in the ocean now so what would be the mechanism for growing the pool you know what's the mechanism for reducing the size of the pool what is it sensitive to so that concludes my presentation thank you so much for joining me if you have questions or comments that you would like to share with me there's my email address and i am very very thankful to the federal agencies who have supported my work for a lot of years now the national science foundation noaa and nasa have all been very very important to my program thank you very much for joining me [Music] hi okay i think we're live now is that correct okay all right so i first like to begin by thanking uh dennis for a really interesting and very informative presentation we now have some time scheduled for questions and answers and if you would would you please enter your questions and answer or your questions rather into the question and answer chat box which should be on the left side of your screen hopefully but we do have one question to start with here and this is from claire reimers and it's for dennis to say can you address the role of doc of terrestrial origin in the global patterns uh hi claire hi elaine yes um you know there are there are laboratories pursuing that question patricia medeiros comes to mind uh it's difficult to find the right tracer that that allows you to move from the tracer concentration to the total traurogenous doc concentrations in for example the deep ocean so uh the tracers are found everywhere for trojans doc but you know we do not know uh what the contribution is ultimately to the total doc concentrations that we're seeing out there it's always been presumed to be low because of isotopic signatures and such but i guess the jury is out as far as just how important is it thank you so please if you would if you have questions for dennis please enter them in the question and answer box on the screen here um i do have a question for you dennis and um while we're waiting for others to to do this um you mentioned about the doc sinks in the pacific and you said the mechanisms are not understood but do you have any speculation as to what that might be well i've thought a lot about it and you know i have not come up with answers i have just logically thought about you know different mechanisms and i've had to dismiss a couple of them but uh you know up in the far north where the the sink exists i thought well maybe there's particles dropping down and maybe there's a residence time of that water mass where the particles can strip doc leaving a little bit of a hole behind but then if you go to the south pacific the sink there is under the uh subtropical gyre so it's it's at about you know the center of it is near 20 degrees south so there's not a lot of particles sinking there to to use that mechanism in both locations um you know we thought about you know our i see a question coming about hydrothermal vents hydrothermal vents than to put out low doc concentrations like matt mccarthy have done that kind of work you know and i remember concentrations of about 11 micro moles per liter of water of doc in the in the vent water but there's not enough vent water to create the you know you think about the volume of these deficits out there they're really large and you don't find other tracers from the vents that coincide appropriately with the uh sync systems so um you know i don't have an answer i i you know i keep thinking is i'm thinking it's probably an abiotic process because i can't think of a reason for a biological mechanism and i'm thinking of you know how do you strip doc abiotically in two completely different systems one underlying the ligatropic system one underlying a more productive system and then you have the mediterranean where there's some low concentrations which are very suggestive of a of a local sink as well and uh you know it's i don't have an answer eileen i wish i did no that's fine that's a fair answer okay um did you want to respond anymore to chris hayes comment about about hydrothermal events as a source of cinco de mayo you know chris i i just you know i look at the same papers and uh you know the hot the hot water vents seem to be low doc and they seem to be as far as i recall radiocarbon dead and so it's just you know that water's been down there for quite a while and and maybe there's some maybe all of the doc that was in the water to make up the vent water was removed and maybe some other organic carbon was added that makes it dead or else it has long resistance but it just seems to be relative to the water going into the vent system from the deep ocean it's lower but again i had i just don't think there's enough volume flux to help us understand the sinks that we see it you know in the midwater column all right we have a thank you there's another question here from david1 and it says the the paleo record indicates that the amount collapses on millennial time scale during the last glacial interval can you speculate on how this would impact the doc distribution in the deep ocean um well if you know i've speculated on that just you know just thinking nothing else to do i think about doc in the deep ocean and if i shut down that circulation and if the system is fairly if that doc is fairly conserved then i can't think of you know if i think about the the transport of those of that recalcitrant dlc from the southern ocean you know along the bottom up to the far north pacific and it's got to conserve behavior and if i turn off the circulation i think it's still going to have a conserved behavior uh you know and so if there's a separation between those sinking particles and the dfc that's being added to the deep waters and supporting the deep microbes if there's a separation from that doc from that background or calcium pool then that recalcular pool is just going to be down there with a much more sluggish circulation and if it's conserved it's conserved so i imagine i i can't imagine a major consequence as far as the change in the concentrations thank you are there any other questions from from our audience [Music] so we don't i don't see any other questions here um let's see okay david lynn says thank you for your answer yeah okay yeah okay do we want to um is there um in the time we have here um we could have a bit of a discussion if we had some questions some questions here um let's see so let me mention just some other things that i i couldn't fit in yeah okay well there is yeah that'll be fine but there is one more question here from from chris hayes which is what our time series sites should what what are time series sites that we should measure uh should we what are the time series sites should we measure doc what are the time series what where we should measure doc oh so where should we add time series create new time series yeah you know the our time series are yeah we think of bats and hot you know where we have the high quality data and you know the pretty large or wide biogeochemical suite of measurements they're not at particularly high latitudes and so uh you know i recognized that as a bias in my my sense it was biasing my sense of dynamics in the deep ocean and so that's why we started doing uh the lion p occupation so we we joined those folks for two to three years of occupations and that's when we saw all kinds of seasonality in the deep ocean it was just crazy seasonality out there and so uh you know three times a year may not you know is it is fine that was all a tip of opportunity and you know let me say unfunded work so uh you know if we could supplement that with some more observations uh whether it was just or station pop or preferably the whole line you know i think they do on the order of eight to twelve stations during each cruise if we do that more than the three times a year you'll come in with another two cruises perhaps that would be great that would be really really good because we can't you know can't add timeshares everywhere but that place is uh you know just amazing signal you know seasonally and i'd really like to understand that a lot better for somebody else somebody else could pick that up because we had to quit you know we just couldn't keep uh keep doing it because of the effort but uh it's important to pick those up great okay thank you um there's a question here from um edward hudson it says you said you hadn't yet had time to comment on the arctic ocean data could you do so now i was surprised to see high surface doc there in one of your plots yeah the the arctic is uh something i i wanted to mention in this phone call or this call so you know the arctic is not not easily accessible and so uh you know early on i i had some measurements from you know north of alaska and that sort of thing but when geotraces came along you know wanted to do a trans arctic section through the north pole i guess that was in 2015 and the u.s side from alaska up to the north pole and from the european side from i guess norway or so up to the north pole on the polar stern i was able to make measurements on the entire section again it was you know unfunded work on the european side but you know we couldn't let the opportunity pass to have a complete look at that system and so uh you know we already knew the surface uh concentrations uh from previous measurements could be high and in fact uh you know there was a paper that we put out from the geotracer's work the lead author was matt charette and that was probably a year or two ago and and that focused on the transpolar drift where we crossed it and i think the concentrations for doc in the transpolar drift well out in the central arctic ocean we're on the other order of 140 micro molar so you know it's pretty pretty darn high you know on the order of twice what you would see even you know lower uh you know subtropical water surface waters so but it's explained you know those higher values simply because you know strong trages input from the rivers especially the russian rivers so you go onto the shelves over on that side of the basin and you'll see pretty high concentrations and it's going to correlate pretty well with salinity lower salinity waters higher dfc concentrations the really uh you know fascinating thing in the arctic is that the concentrations in the deep basins especially the eurasian basin but also the deep canada basin those concentrations in the eurasian basin are approaching 50 micro moles per kilogram and those waters have a residence time of a few hundred years so if if we took water in the north atlantic and you know send it off you know with overturned circulation set it south within about you know some few decades a lot of that carbon is going to get knocked back we saw that there's a residual you could see it all the way in the indian ocean but you know the microbes are going to do their job and take up what they can but we take that same water and we send it down to the deep eurasian basin and we give it hundreds of years of residence time it's still 50 micromolar that's about the concentration of that north atlantic water when it comes in that's what it's bringing into the system so uh it's an amazing uh feature and even the deep canada basin which isn't you know another couple hundred years in residence time is still up in the mid 40 micromolar range you know which is a lot higher than we would see anywhere except for the far north atlantic and again the far north atlantic would be much more recently formed water masses so it's an intriguing feature that uh you know i'm working with rob fletcher and bill meppe and andrew margolin trying to figure that out and you know i i think we're kind of leaning towards those enrichments perhaps being someone asked earlier about the george's doc i think maybe eileen did huh about the treasures doc and you know it could be that just terrigenous carbon down there you know ron benner's done work down deep looking at lignin concentrations and ellen druffles looked at c-14 content of the organic matter down there and there's enough pieces of the puzzle that might come together to say that you know it could just be terrigious dlc in richmond but it would be on the order of 10 micromolar you know relative to the 50 down there i would think it'd have to be that much with the other 40 being marine so uh it's something that we're really interested in trying to understand because it's really nice now we've also done work in the black sea and the black deep black sea has a lot of you know we think tourist doc so that's what made us think terrigenous in the arctic ocean as well so it could be that you know there's a lot maybe a lot more down the deep ocean than we understand but we can see it in the deep arctic ocean perhaps we have a proof of that we're just speculating but we're yeah we know the distribution is what it is which is fascinating but we don't have the proof yet as far as tracers telling us what that composition is okay um thank you there there's a follow-on question sort of here from david demarisis has there been enough time since the loss of north polar ice to document its effects on doc and if so what might be the facts i didn't catch the first part of the question is there enough time has there been enough time since the loss of north polar ice to document its effects on doc well you know anytime we're looking at surface waters then biology you know there's a lot of variability just from local biological processes so the the primary effect of melting ice up there is going to be to dilute those surface doc concentrations so we see that you know in a retreating ice edge you know so for example when we did the u.s side of the geotraces work whenever we got into some more recently uh melted ice system you know so we you know got that little fresh fresher water lens at the surface the doc concentrations were low relatively low because of dilution from the very low doc sea ice melting and just diluting the the sea water which would have higher doc concentrations so that would be one effect now again you know so that would be more of a you know to the extent that that dilution is uh you know a more regular feature then you're going to more regularly be diluting your doc concentrations now if you add biology on top of it you know some bile you know some bloom takes place it's going to add some dlc microbes are going to pull it down so there's that seasonality still that i would expect that would kind of make you know make it a little bit harder to say that there's uh you know to be able to to see a role for that sea ice clearance except that you know there's more sunlight reaching that surface so maybe some more doc okay well sort of following with the um the global change types of ideas there's a question here from claire reimers is that uh what are current projections for ncp in different regions with global warming uh you know i've eileen you probably know better than me at the answer that question i mean sometimes i see the winds are going to strengthen upwelling in some places and you know warming's going to increase stratification in other places and i don't know what the net effect is on that but you know we uh wherever you have an enrichment of ncp you know enhancement of ncp then you're going to have that's where the that's where the doc production is taking place you know so globally you got to go to those locations for you know the divergent zone so to the extent that they're enhanced by winds then you know you're going to get the enrichment to the extent that there's more stratification somewhere it's going to be shut down i don't know what that what the end result is globally be interesting to watch it unfold here no no um well while we're waiting for some other questions dennis did you want to say you mentioned that you might have you wanted to say something about some of the things you weren't able to incorporate into your presentation do you have it just i think the the arctic is really what i wanted to talk about i've done that um now you know i if bill rieberg is on the line he should be posing the question too but bill on the spot here yeah i'm not sure he's online but all right all right do we have any other um questions i think i don't think i've missed any here um i think let me just say for you know people you know younger scientists there's a lot of opportunity out there for figuring this pool out still you know you know i took the low hanging fruit you know i figure out global distributions uh you know some temporal variability try to understand some fundamental nature of the dynamics of it you know where is it being produced how much is being produced but but again we don't know mechanisms who's responsible uh you know and if we don't understand mechanisms mechanisms will never understand sensitivity so you know we don't we don't understand is there's some work going on and trying to understand the persistence of the recalcitrant pool what is it that allows that deep ocean doc to persist for as many thousands of years as it does you know is it is it intrinsically difficult to break down for microbes or or is there molecular diversification so great that individual uh molecules are just incredibly low concentrations and therefore not accessible to the microbes there's all kinds of great questions that need to be that are being pursued and that need to be pursued so i hope uh some people will uh take that opportunity that's a good comment yeah that for to understand that it's not all done yet and there are many many more questions still out there for for people to look at with this and very important questions as you've pointed out in your presentation so thank i want to thank everyone who tuned in today and i really appreciate it and and thank you claire and eileen as well yeah and i would like to thank you for for your presentation and for um everyone our audience here for accommodating the first virtual online agu meeting and session it's [Music] been an interesting exercise with the covet 19 and the virtual meetings but it seems to have worked well and like to thank the ocean sciences program committee members who set all this up and also the agu staff who have supported the development of this and and made sure that it worked in in a way that was very convenient and relatively easy for everyone to use so there are many thanks here to people who have made the effort for us to have a fall agu meeting this year and we'll thank them all anyway color do you want to say anything before we end here in my life yes you are okay um just i echo your um thanks to particularly the aku staff for making this possible and remind folks that these lectures are being recorded and will be available for for viewing in in the future and so you can tell your colleagues um that they can still uh take part in the lecture so thank you all right and one last reminder here is that we have ocean sciences has two additional named lectures it will be the rachel carson lecture on thursday at um 1 30 eastern time being given by carvel from cornell and then the sphere drift lecture on friday again 1 30 eastern time being given by arnold gordon i hope you all make time for those later in this week okay so i think we're almost at the end of our time here if there's no other questions or anything i think we can close the session okay all right but again thank you to dennis and everyone for your participation right thank you thank you