William and Carolyn Reiberg Lecture by Dr. Dennis Hansel

Introduction by Claire Reimers

Dennis Hansel is a professor at the University of Miami, Rosenstiel School of Marine and Atmospheric Science.
Known for his work on dissolved organic matter (DOM) and its role in the carbon and nitrogen cycles in the ocean.
Earned his PhD at the University of Alaska in Fairbanks, worked on the nitrogen cycle in the Bering and Chukchi seas.
Has contributed over 100 academic articles and two editions of "Biogeochemistry of Marine Dissolved Organic Matter".
Honored with AGU's Harold Sphere to Fletcher and named an AGU Fellow and AAAS Fellow.
Chairs and trustees various boards and councils related to ocean sciences.
Attuned to research despite COVID-19; enjoys morning bike rides.
Lecture title: "Oceanic Dissolved Organic Carbon: The World Tour" with a Q&A session after.

Terrestrial and oceanic carbon reservoirs are significant components of Earth's carbon system.
Soil Organic Carbon (upper layers of soil) vs. Dissolved Organic Carbon (DOC) in the ocean.
Breakdown of organic matter leading to DOC in both types of environments.
Comparison between the carbon reservoirs in soil (2000 petagrams) and ocean (660 petagrams).
Distribution patterns and dynamics of DOC in various ocean basins.

Global sections (Atlantic, Pacific, Indian Oceans) showing DOC distribution: surface enrichment, lower concentrations at depth.
Concentration color-coded: blues/pinks (low), greens (intermediate), reds (high).
DOC accumulation in the euphoric zone (produced by autotrophs), modern carbon, and older carbon.
Surface DOC linked to the main thermocline in the ocean.

Biological production of DOC, seasonal variability observed (e.g., Bermuda Atlantic Time Series Study - BATS).
Relationship between net community production (NCP) and DOC accumulation.
Studies in various regions like the Gulf of Alaska and Atlantic with data from biogeochemical Argo floats and time series.

DOC transported to depth through North Atlantic Deep Water and circumpolar pathways.
Salinity as a tracer to understand DOC distributions better, revealing conserved and non-conserved segments of DOC.
Abiotic and biotic processes might play a role in DOC dynamics.

Observations from the South Atlantic, Indian Oceans confirming transport mechanisms of DOC.
Additional studies on deep water dynamics in the Pacific, showing enrichment and removal zones for DOC.
Mixing and conservation of DOC at different depths and geographical locations.

Identified regional sinks of DOC (e.g., North Pacific, South Pacific) with debated mechanisms behind these sinks.
Discussed the importance of understanding these processes to predict future DOC dynamics and their sensitivities.

Importance of process understanding to predict DOC changes with climate change.
Outstanding research questions regarding mechanisms driving the persistence and removal of DOC in ocean systems.

Importance of ongoing and future research in understanding DOC dynamics and its implications on the global carbon cycle.
Acknowledgment of supporting agencies and collaborators.

Role of terrestrial DOC: - Ongoing laboratory investigations; jury is out on the ultimate contribution to total oceanic DOC.
DOC sinks in the Pacific: - Speculated on abiotic processes but recognized lack of concrete mechanisms.
Hydrothermal vents: - Discussed vent water characteristics but concluded insufficient volume to impact DOC sinks significantly.
Global warming impacts on NCP and DOC: - Comments on the complexity of changes with enhanced upwelling and increased stratification.
Arctic Ocean dynamics: - High surface DOC due to riverine inputs, persisting high deep-water DOC concentrations due to unknown mechanisms.