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The sinking of particulate matter from the upper ocean dominates the export and sequestration of organic carbon by the biological pump, a critical component of the Earth's carbon cycle. Controls on carbon export are thought to be driven by ecological processes that produce and repackage sinking biogenic particles. Here, we present observations during the demise of the Northeast Atlantic Ocean spring bloom illustrating the importance of storm-induced turbulence on the dynamics of sinking particles. A sequence of four large storms caused upper layer mean turbulence levels to vary by more than three orders of magnitude. Large particle (>0.1 to 10 mm) abundance and size changed accordingly: increasing via shear coagulation when turbulence was moderate and decreasing rapidly when turbulence was intense due to shear disaggregation. Particle export was also tied to storm forcing as large particles were mixed to depth during mixed layer deepening. After the mixed layer shoaled, these particles, now isolated from intense surface mixing, grew larger and subsequently sank. This sequence of events matched the timing of sinking particle flux observations. Particle export was influenced by increases in aggregate abundance and porosity, which appeared to be enhanced by the repeated creation and destruction of aggregates. Last, particle transit efficiency through the mesopelagic zone was reduced by presumably biotic processes that created small particles (<0.5 mm) from larger ones. Our results demonstrate that ocean turbulence significantly impacts the nature and dynamics of sinking particles, strongly influencing particle export and the efficiency of the biological pump.

This manuscript presents an overview of NASA's EXport Processes in the Ocean from Remote Sensing 2021 Field Campaign in the North Atlantic (EXPORTS NA) and provides quantitative and dynamical descriptions of the physical processes modulating water transformations during the study. A major programmatic goal was to conduct the sampling in a Lagrangian mode so that ocean ecological and biogeochemical changes can be observed independent from physical advective processes. To accomplish this goal, EXPORTS NA conducted a multi-ship, multi-asset field sampling program within a retentive, anticyclonic mode water eddy. Beneath depths of ∼ 100 m, Lagrangian sampling assets remained within the eddy core waters (ECWs) throughout the experiment, demonstrating that the ECWs within the mode water eddy were retentive. However, strong westerly winds from four storm events deepened the mixed layer (ML) of the surface core waters (SCWs) above the eddy's mode water core by 25–40 m and exchanged some of the SCWs with surface waters outside of the eddy via Ekman transport. Estimates of flushing times ranged from 5 to 8 days, with surface exchange fractions ranging from 20 to 75 % and were consistent with particle tracking advected by combined geostrophic and Ekman velocities. The relative contributions of horizontal and vertical advection on changes in ECW tracers depended on the horizontal and vertical gradients of that tracer. For example, horizontal advection played a large role in ECW salinity fluxes, while vertical entrainment played a larger role in the fluxes of nutrients into SCW ML. Each storm injected nutrients and low oxygen waters into the ML, after which the surface ocean ecosystem responded by reducing nutrient concentrations and increasing %O2 saturation levels. Overall, ECW values of chlorophyll and POC were the largest at the onset of the field program and decreased throughout the campaign. The analysis presented provides a physical oceanographic context for the many meas

This manuscript presents an overview of NASA’s EXport Processes in the Ocean from Remote Sensing 2021 Field Campaign in the North Atlantic (EXPORTS NA) and provides quantitative and dynamical descriptions of the physical processes modulating water transformations during the study. A major programmatic goal was to conduct the sampling in a Lagrangian mode so that ocean ecological and biogeochemical changes can be observed independent from physical advective processes. To accomplish this goal, EXPORTS NA conducted a multi-ship, multi-asset field sampling program within a retentive, anticyclonic mode water eddy. Beneath depths of ∼100 m, Lagrangian sampling assets remained within the eddy core waters (ECWs) throughout the experiment, demonstrating that the ECWs within the mode water eddy were retentive. However, strong westerly winds from four storm events deepened the mixed layer (ML) of the surface core waters (SCWs) above the eddy’s mode water core by 25-40 m and exchanged some of the SCWs with surface waters outside of the eddy via Ekman transport. Estimates of flushing times ranged from 5-8 days, with surface exchange fractions ranging from 20-75% and were consistent with particle tracking advected by combined geostrophic and Ekman velocities. The relative contributions of horizontal and vertical advection on changes in ECW tracers depended on the horizontal and vertical gradients of that tracer. For example, horizontal advection played a large role in ECW salinity fluxes, while vertical entrainment played a larger role in the fluxes of nutrients into SCW ML. Each storm injected nutrients and low oxygen waters into the ML, after which the surface ocean ecosystem responded by reducing nutrient concentrations and increasing %O2 saturation levels. Overall, ECW values of chlorophyll and POC were the largest at the onset of the field program and decreased throughout the campaign. The analysis presented provides a physical oceanographic context for the many measurements

EXport Processes in the Ocean from Remote Sensing (EXPORTS) is a large-scale NASA-led and NSF co-funded field campaign that will provide critical information for quantifying the export and fate of upper ocean net primary production (NPP) using satellite information and state of the art technology.

Dataset: Dissolved trace metal (Mn, Fe, Co, Ni, Cu, Zn, Cd, Pb) concentration, Concentrations of dissolved (<0.2 µm) manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), cadmium (Cd), lead (Pb) in surface (~2 m) samples collected between 15 August 2018 and 6 September 2018 during EXports Processes in the Oceans from RemoTe Sensing (EXPORTS) cruise aboard RV Roger Revelle RR1813 at Ocean Station PAPA (Station P). For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/869683, NSF Division of Ocean Sciences (NSF OCE) OCE-1756816, NSF Division of Ocean Sciences (NSF OCE) OCE-1756433, NSF Division of Ocean Sciences (NSF OCE) OCE-1756442

Diatoms serve as a critical link between the marine carbon (C), nitrogen (N), and silicon (Si) biogeochemical cycles through an obligate requirement for silicon for frustule formation. The siliceous frustules of diatoms serve as ballast, leading to the disproportionate contribution of diatoms to organic carbon export compared to similarly sized non-siliceous cells. Despite accounting for ~30% of marine primary production, diatom growth and productivity are limited in much of the surface ocean by some combination of low concentrations of dissolved iron (dFe), nitrate (NO3-), and silicic acid (Si(OH)4). Furthermore, as obligate phototrophs living in the sunlight region of the ocean, diatom cells can become increasingly light-limited as light quality and quantity decreases over depth. These physiological limitations can lead to shifts in phytoplankton community composition and act as strong controls on phytoplankton physiology, marine biogeochemical cycling, and carbon export.With the growing abundance of marine nucleic acid sequencing data through large collaborative projects such as EXPORTS, BIOS-SCOPE, BioGEOTRACES, Global Ocean Sampling, Tara Oceans, and Bio-GO-SHIP, which all share an interest in moving toward linking cellular metabolism with ocean processes, there is a need to develop and test relationships between molecular markers, phytoplankton physiology, and related biogeochemical stocks and rates. By relating metatranscriptomic patterns to biogeochemical stock and rate measurements during diatom blooms in the California Upwelling Zone we evaluate the ecological relevance of diatom transcriptomic fingerprints of iron, nitrogen, silicon, and light limitations based on those previously characterized in laboratory studies, while also exploring potentially undiscovered Si related genes. The first chapter of this dissertation (Chapter 1) explores the transcriptomic and physiological changes that occur in iron limited diatom assemblages that lead to bulk increases

Diatoms are known to fractionate silicon isotopes during the formation of their frustules causing the silicon isotopic composition of biogenic silica to track the degree of silicic acid consumption in surface waters. Despite a growing body of work that uses this proxy to reconstruct past changes in silicic acid utilization, the understanding of the benthic silicon cycle, particularly the identification and quantification of the processes that potentially alter the silicon isotopic composition of biogenic silica during early diagenesis is still lacking. We investigated these processes by comparing the silicon isotopic composition of pore water silicic acid, biogenic silica and, for the first time, lithogenic silica from five sediment cores collected in the deep basin of the Southern Ocean representing a diversity of sedimentation regimes. Silicic acid concentrations and the isotopic composition of Southern Ocean pore waters were the result of a dynamic balance between the dissolution of biogenic silica, reactive lithogenic silica phases and Si re-precipitation with the relative importance of each processes differing significantly between regions. The results are consistent with the formation of authigenic alumino-silicates derived from dissolved biogenic silica in the Sub-Antarctic Zone and in the Antarctic Zone (on average 12 ± 5% and 17 ± 13%, respectively). Since this latter process can fractionate silicon isotopes, this implies that, even if the silicon isotopic composition of diatoms preserved in the sediments is a reliable proxy for silicic acid utilization in the past ocean, care must be taken to extract a clean biogenic silica phase free of authigenic clays and lithogenic phases from sediments to eliminate this potential bias when interpreting isotopic records.

Diatoms are major contributors to marine primary productivity and carbon export due to their rapid growth in high-nutrient environments and their heavy silica ballast. Their contributions are highly modified in high-nutrient low-chlorophyll regions due to the decoupling of upper-ocean silicon and carbon cycling caused by low iron (Fe). The Si cycle and the role of diatoms in the biological carbon pump was examined at Ocean Station Papa (OSP) in the HNLC region of the northeastern subarctic Pacific during the NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field study. Sampling occurred during the annual minimum in surface silicic acid (Si(OH)4) concentration. Biogenic silica (bSi) concentrations were low, being in the tens of nanomolar range, despite high Si(OH)4 concentrations of about 15 mM. On average, the >5.0-mm particle size fraction dominated Si dynamics, accounting for 65% of bSi stocks and 81% of Si uptake compared to the small fraction (0.6-5.0 mm). Limitation of Si uptake was detected in the small, but not the large, size fraction. Growth rate in small diatoms was limited by Fe, while their Si uptake was restricted by Si(OH)4 concentration, whereas larger diatoms were only growth-limited by Fe. About a third of bSi production was exported out of the upper 100 m.The contribution of diatoms to carbon export (9-13%) was about twice their contribution to primary productivity (3-7%). The combination of low bSi production, low diatom primary productivity and high bSi export efficiency at OSP was more similar to the dynamics in the subtropical gyres than to other high-nutrient low-chlorophyll regions.