Processes controlling the distribution of biogeochemical tracers in the ocean

Abstract

This thesis investigates the mechanisms controlling ocean biogeochemical tracers, including ocean circulation, nutrient and carbon cycles, and the marine ecosystem.

In Chapter 2, I focus on the distribution of helium isotopes in the ocean. The high 3He/4He ratio of oceanic helium relative to the atmosphere is a signature of mantle 3He outgassing from the Earth's interior. This flux is commonly used to estimates chemical fluxes of elements from the solid Earth, and to constrain models of mantle dynamics. Here, I use a suite of ocean general circulation models and helium isotope data from the World Ocean Circulation Experiment to revise the flux of helium from the mantle to the oceans. I show that a flux of 527±102 mol/year, approximately 2 times smaller than previous estimates, is required to match observed helium in ocean general circulation models calibrated against observations of ventilation tracers. This new estimate calls for a revision of the element fluxes that are currently linked to helium outgassing, including noble gases and carbon dioxide.

In Chapter 3, I investigate the distribution and biogeochemical processes that characterize open ocean Oxygen Minimum Zones (OMZs). OMZs are major sites of fixed nitrogen removal from the ocean, and a net source of nitrous oxide, a powerful greenhouse gas. First, I provide a revised estimate of OMZ distribution based on analysis and synthesis of in situ measurements. The resulting suboxic volumes (O2<5-10 mmol/m3 ) are a factor 3 larger than commonly used datasets. Second, I combine the new oxygen data sets with estimates of global export and simple models of remineralization to estimate global denitrification and N2O production. The results (1) reconcile water column denitrification rates with existing estimates based on nitrogen isotopes, (2) revise previous estimates of sediment denitrification down by 1/3, and (3) independently support a historically-balanced oceanic nitrogen cycle. I explore the sensitivity of the estimates to the model parameters and quantify the effects of ocean deoxygenation, an expected response to anthropogenic climate change.

In Chapter 4, I investigate the controls and impacts of diel vertical migration (DVM) of zooplankton and fish on water column ecosystem and biogeochemistry. DVM is a fundamental component of the biological pump, but is generally overlooked in global models of the Earth System. I develop a representation of DVM in a size-structured NPZD model and evaluate it against data from three major biomes in the Pacific Ocean. The model includes: (1) a realistic parameterization of migration dynamics, (2) a representation of the processes that decouple zooplankton feeding and respiration in the water column, and (3) a consistent parameterization of visual predation. The model captures to first order the biomass and nutrient flux partitioning between different trophic levels, and the proportion of migrating zooplankton. I show that realistic migratory populations sustain active fluxes of between 20 and 40 % of the particle export to the mesopelagic zone, and contribute up to 50 % of the total respiration within the layers affected by migration. This has consequences for oxygen, nutrient, and carbon cycling. I highlight the importance of allowing zooplankton feeding and excretion to vary independently with depth - a process that should be included in global parameterizations of DVM.