Phoebe J. Lam

The rapidly changing Arctic Ocean remain poorly constrained with respect to particle dynamics and particulate organic carbon export. In cloudy and ice-covered polar regions, in-situ measurements are often the only means of assessing the biological carbon pump throughout most of the year. Here we synthesize high-resolution upper ocean observations of ²³⁴ Th (t _½ ~ 24 days) during an expansive GEOTRACES collaboration involving two 2015 expeditions spanning from the Bering Strait to the Barents Sea from late summer-fall 2015. This effort confirms that vertical surface export fluxes in the central Arctic basin (>1000 m water depth) continue to remain extremely low (range: 0–4 mmolC m ^-2 d ^-1 ). Conversely, the shelf-slope regions exhibited stronger and more variable Th deficits, with short (days to 2 weeks) residence times for dissolved ²³⁴ Th and an order of magnitude higher POC export, on average (range: 1–77 mmolC m ^-2 d ^-1 ). These shallow shelf fluxes reflect enhanced particle scavenging but may be biased by sediment resuspension, lateral advection and non-steady-state dynamics. This international effort provides an important baseline for the state of the Arctic biological pump as it is the last of only three campaigns (2005–2025) to report ²³⁴ Th-based export estimates from the central Arctic basin. Regular monitoring of Arctic Ocean is paramount as ice cover continues to decline, and more frequent episodic export events, large under ice blooms, and increased lateral transport from Arctic shelves have the potential to significantly impact the amount of particulate carbon buried in shelf sediments and reaching the deep Arctic.

The concentrations of particulate organic carbon (POC) in the ocean are the result of a dynamic balance between multiple sources and sinks. Here, measurements of POC concentration in the small size fraction (SSF; 1–5 ) and the large size fraction (LSF; ) from the North Atlantic EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) cruise in the Porcupine Basin are combined with a non‐steady state POC cycling model to estimate the temporal variations in rates of POC production, (dis)aggregation, sinking, remineralization, and POC ingestion/egestion by migrating zooplankton over the decline of a phytoplankton bloom. We infer coherent variations in POC cycling rates throughout the water column, with distinct temporal variability. Throughout the upper 500 m, disaggregation exerts a major control on both size fractions. Near the surface, disaggregation rates increase with time and are the major source of POC in the SSF. Aggregation strongly influences surface POC in both size fractions, although it becomes negligible with depth. POC is supplied to the LSF between 175 and 500 m by migrating zooplankton, which contribute between 15% and 100% of the total LSF source in this depth interval. Our results highlight a need to improve estimates of POC sinking fluxes, stressing differences in multiple approaches. In general, we quantify the intensity of particle cycling processes that are hard to observe and identify key mechanisms that lead to the observed temporal and vertical variations in POC concentrations.

The concentrations of particulate organic carbon (POC) in the ocean are the result of a dynamic balance between multiple sources and sinks. Here, measurements of POC concentration in the small size fraction (SSF; 1–5 μm) and the large size fraction (LSF; > 5 μm) from the North Atlantic EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) cruise in the Porcupine Basin are combined with a non-steady state POC cycling model to estimate the temporal variations in rates of POC production, (dis)aggregation, sinking, remineralization, and POC ingestion/egestion by migrating zooplankton over the decline of a phytoplankton bloom. We find coherent variations in POC cycling rates throughout the water column, with distinct temporal variability. Throughout the upper 500 m, disaggregation exerts a major control on both size fractions. Near the surface, disaggregation rates increase with time and are the major source of POC in the SSF. Aggregation strongly influences on surface POC in both size fractions, although it becomes negligible with depth. POC is supplied to the LSF between 330 m and 500 m by migrating zooplankton, which contribute between 75% and 97% of the total LSF source in this depth interval. Our inversion highlights a need to improve estimates of POC sinking fluxes, highlighting differences in multiple approaches. In general, we quantify the intensity of particle cycling processes that are hard to observe and identify key mechanisms that lead to the observed temporal and vertical variations in POC concentrations.

The biological pump, a fundamental process governing atmospheric CO₂, rapidly transfers particulate inorganic and organic carbon (PIC and POC) from surface waters to the deep sea but is inherently highly variable in space and time, and thus poorly observed. Here we synthesize PIC and POC data from satellites, CTD‐profiled optical sensors (birefringence and transmissometer), and from in situ pumps samples from GEOTRACES transects spanning 20,000 km from the North Pacific to Southern Ocean. High resolution profile data from PIC sensors revealed strong subsurface maxima in the deepest euphotic zone waters of oligotrophic gyres; furthermore, data showed high concentrations of PIC penetrating to >500 m south of the Subarctic Front (45°N–35°N), at the equator, and north of the Antarctic Polar Front (45°S–55°S) indicating high carbon export in these regions. We developed a new temporal/spatial interpolation scheme for satellite data that improved matchups with ship observations. North of the Antarctic Polar Front (APF), PIC sensor data was generally well aligned with sample PIC; however, a positive bias of satellite PIC was found in poor retrieval regions. South of the APF, both satellite and birefringence sensor greatly overestimated PIC by factors of >25 and 12, respectively, compared to sample PIC which averaged 15 nM. The unanticipated discovery of a non‐carbonate particle birefringence source coupled with a microscopic investigation of pump samples leads us to hypothesize that internal reflection within bubbles and/or cellular structures of heavily silicified colony‐forming diatoms (Fragilariopsis and Pseudo‐nitzschia) is the cause for anomalous birefringence and adds to backscattered satellite radiances.

The physical and biogeochemical properties of the western Arctic Ocean are rapidly changing, resulting in cascading shifts to the local ecosystems. The nutrient‐rich Pacific water inflow to the Arctic through the Bering Strait is modified on the Chukchi and East Siberian shelves by brine rejection during sea ice formation, resulting in a strong halocline (called the Upper Halocline Layer (UHL)) that separates the cold and relatively fresh surface layer from the warmer and more saline (and nutrient‐poor) Atlantic‐derived water below. Biogeochemical signals entrained into the UHL result from Pacific Waters modified by sediment and river influence on the shelf. In this synthesis, we bring together data from the 2015 Arctic U.S. GEOTRACES program to implement a multi‐tracer (dissolved and particulate trace elements, radioactive and stable isotopes, macronutrients, and dissolved gas/atmospheric tracers) approach to assess the relative influence of shelf sediments, rivers, and Pacific seawater contribution to the Amerasian Arctic halocline. For each element, we characterized their behavior as mixing dominated (e.g., dCu, dGa), shelf‐influenced (e.g., dFe, dZn), or a combination of both (e.g., dBa, dNi). Leveraging this framework, we assessed sources and sinks contributing to elemental distributions: shelf sediments (e.g., dFe, dZn, dCd, dHg), riverine sources, (e.g., dCu, dBa, dissolved organic carbon), and scavenging by particles originating on the shelf (e.g., dFe, dMn, dV, etc.). Additionally, synthesized results from isotopic and atmospheric tracers yielded tracer age estimates for the Upper Halocline ranging between 1 and 2 decades on a spatial gradient consistent with cyclonic circulation.

An important role in the cycling of marine trace elements is scavenging, their adsorption and removal from the water column by sinking particles. Boundary scavenging occurs when areas of strong particle flux drive preferential removal of the trace metals at locations of enhanced scavenging. Due to its uniform production and quick burial via scavenging, ²³⁰Th is used to assess sedimentary mass fluxes; however, these calculations are potentially biased near regions where net lateral transport of dissolved ²³⁰Th violates the assumption that the flux of particulate ²³⁰Th to the seabed equals its rate of production in the water column. Here, we present a water column transect of dissolved ²³⁰Th along 152° W between Alaska and Tahiti (GEOTRACES GP15), where we examine ²³⁰Th profiles across multiple biogeochemical provinces and, novelly, the lateral transport of ²³⁰Th to distal East Pacific Rise hydrothermal plumes. We observed a strong relationship between the slope of dissolved ²³⁰Th concentration‐depth profiles and suspended particle matter inventory in the upper‐mid water column, reinforcing the view that biogenic particle mass flux sets the background ²³⁰Th distribution in open ocean settings. We find that, instead of the region of enhanced particle flux around the equator, hydrothermal plumes act as a regional boundary sink of ²³⁰Th. At 152° W, we found that the flux‐to‐production ratio, and thereby error in ²³⁰Th‐normalized sediment flux, is between 0.80 and 1.50 for hydrothermal water, but the error is likely larger approaching the East Pacific Rise.

The biological pump, a fundamental process governing atmospheric CO2, rapidly transfers particulate inorganic and organic carbon (PIC and POC) from surface waters to the deep sea but is inherently highly variable in space and time, and thus poorly observed. Here we synthesize PIC and POC data from satellites, CTD-profiled optical sensors (birefringence and transmissometer), and from in-situ pumps samples from GEOTRACES transects spanning 20,000 km from the North Pacific to Southern Ocean. High resolution profile data from PIC sensors revealed strong subsurface maxima in the deepest euphotic zone waters of oligotrophic gyres; furthermore, data showed high concentrations of PIC penetrating to >500 m south of the Subarctic Front (45°N – 35°N), at the equator, and north of the Antarctic Polar Front (45°S – 55°S) indicating high carbon export in these regions. We developed a new interpolation scheme for satellite data that improved matchups with ship observations. North of the Antarctic Polar Front (APF), PIC sensor data was generally well aligned with sample PIC; however, a positive bias of satellite PIC was found in poor retrieval regions. South of the APF, both satellite and birefringence sensor greatly overestimated PIC by factors of >25 and 12, respectively, compared to sample PIC which averaged 15nM. The unanticipated discovery of a strong non-carbonate particle birefringence source coupled with a microscopic investigation of pump samples leads us hypothesize that internal reflection within bubbles and/or body structures of heavily silicified colony-forming diatoms (Fragilariopsis and Pseudo-nitzschia) is the cause for both anomalous birefringence and backscattered satellite radiances.

Observations of dissolved cadmium (dCd) and phosphate (PO₄) suggest an unexplained loss of dCd to the particulate phase in tropical oxyclines. Here, we compile existing observations of particulate Cd and phosphorus (P), and present new data from the US GEOTRACES GP15 Pacific Meridional Transect to examine this phenomenon from a particulate Cd perspective. We use a simple algorithm to reproduce station depth profiles of particulate Cd and P via regeneration and possible subsurface accumulation. Our examination of regeneration reveals decoupling of particulate Cd and P driven by variable partitioning between two particulate pools with differing labilities. Further, we identify evidence for subsurface particulate Cd accumulation at 31 stations. Subsurface particulate Cd accumulation occurs most consistently in the mesopelagic tropical Pacific but can be found in all examined ocean basins. This accumulation is not well‐correlated with dissolved oxygen or particulate sulfide concentration. Instead, we observe that particulate Cd accumulation occurs in regions where the concentration of dCd is relatively high compared to dissolved zinc (dZn) and speculate that it is the result of enhanced dCd biological uptake in response to the subsurface micronutrient balance.

Distributions of the natural radionuclide ²¹⁰Po and its grandparent ²¹⁰Pb along the GP15 Pacific Meridional Transect provide information on scavenging rates of reactive chemical species throughout the water column and fluxes of particulate organic carbon (POC) from the primary production zone (PPZ). ²¹⁰Pb is in excess of its grandparent ²²⁶Ra in the upper 400–700 m due to the atmospheric flux of ²¹⁰Pb. Mid‐water ²¹⁰Pb/²²⁶Ra activity ratios are close to radioactive equilibrium (1.0) north of ∼20°N, indicating slow scavenging, but deficiencies at stations near and south of the equator suggest more rapid scavenging associated with a “particle veil” located at the equator and hydrothermal processes at the East Pacific Rise. Scavenging of ²¹⁰Pb and ²¹⁰Po is evident in the bottom 500–1,000 m at most stations due to enhanced removal in the nepheloid layer. Deficits in the PPZ of ²¹⁰Po (relative to ²¹⁰Pb) and ²¹⁰Pb (relative to ²²⁶Ra decay and the ²¹⁰Pb atmospheric flux), together with POC concentrations and particulate ²¹⁰Po and ²¹⁰Pb activities, are used to calculate export fluxes of POC from the PPZ. ²¹⁰Po‐derived POC fluxes on large (>51 μm) particles range from 15.5 ± 1.3 mmol C/m²/d to 1.5 ± 0.2 mmol C/m²/d and are highest in the Subarctic North Pacific; ²¹⁰Pb‐derived fluxes range from 6.7 ± 1.8 mmol C/m²/d to 0.2 ± 0.1 mmol C/m²/d. Both ²¹⁰Po‐ and ²¹⁰Pb‐derived POC fluxes are greater than those calculated using the ²³⁴Th proxy, possibly due to different integration times of the radionuclides, considering their different radioactive mean‐lives and scavenging mean residence times.

The international GEOTRACES program has been instrumental in demonstrating how marine sediments are a critical source of dissolved Fe to the world's oceans. Here, we present dissolved iron (dFe) from the GEOTRACES North Pacific GP15 section, which, alongside other sediment‐source tracers (including dissolved δ⁵⁶Fe, Mn, ²²⁸Ra, and particulate Fe), allows for identification of the dFe provenance of three distinct dFe depth maxima at the Alaskan margin. Two of these (shelf and abyssal depths) are of local Alaskan sedimentary origin. The third, a mid‐depth dFe maximum with an absence of ²²⁸Ra, is an advected signal that, based on tracer data from Western Pacific GEOTRACES transects and circulation models, must be advected from sedimentary sources on the Asian margin, ∼5,000 km away. This study illustrates the importance of measuring diagnostic sedimentary tracers like radium when assigning local margins as sedimentary sources of marine trace metal budgets.