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We present concentrations of total particulate carbon (PC), carbonate (PIC), total particulate nitrogen (PN), δ¹³C and δ¹⁵N of suspended particles along a North Pacific transect. In the upper 400 m, suspended PIC to particulate organic carbon (POC) ratios generally follow published sinking PIC/POC ratios. Below 600 m, suspended PIC/POC become significantly lower than sinking PIC/POC, likely indicating PIC dissolution within the suspended load. In three out of the five stations, suspended PN δ¹⁵N increase with depth from the euphotic zone to the thermocline, consistent with previous observations in many ocean regions (e.g.,BATS and HOT). However, in the other two stations where phytoplankton blooms were encountered, high suspended PN δ¹⁵N (up to 12‰) were observed in the euphotic zone, which was likely caused by the export of low-δ¹⁵N PN during the phytoplankton blooms. Average C_(org) : N ratio of suspended particles along the transect is 5.1 ± 0.2, with the value in the subtropical gyre (5.8 ± 0.3) slightly higher than the subarctic gyre (4.6 ± 0.2).

The deep ocean has long been recognized as the reservoir that stores the carbon dioxide (CO₂) removed from the atmosphere during Pleistocene glacial periods. The removal of glacial atmospheric CO₂ into the ocean is likely modulated by an increase in the degree of utilization of macronutrients at the sea surface and enhanced storage of respired CO₂ in the deep ocean, known as enhanced efficiency of the biological pump. Enhanced biological pump efficiency during glacial periods is most easily documented in the deep ocean using proxies for oxygen concentrations, which are directly linked to respiratory CO₂ levels. We document the enhanced storage of respired CO₂ during the Last Glacial Maximum (LGM) in the Pacific Southern Ocean and deepest Equatorial Pacific using records of deglacial authigenic manganese, which form as relict peaks during increases in bottom water oxygen (BWO) concentration. These peaks are found at depths and regions where other oxygenation histories have been ambiguous, due to diagenetic alteration of authigenic uranium, another proxy for BWO. Our results require that the entirety of the abyssal Pacific below approximately 1,000 m was enriched in respired CO₂ and depleted in oxygen during the LGM. The presence of authigenic Mn enrichment in the deep Equatorial Pacific for each of the last five deglaciations suggests that the storage of respired CO₂ in the deep ocean is a ubiquitous feature of late-Pleistocene ice ages.

Particulate organic carbon (POC) produced in the surface ocean sinks through the water column and is respired at depth, acting as a primary vector sequestering carbon in the abyssal ocean. Atmospheric carbon dioxide levels are sensitive to the length (depth) scale over which respiration converts POC back to inorganic carbon, because shallower waters exchange with the atmosphere more rapidly than deeper ones. However, estimates of this carbon regeneration length scale and its spatiotemporal variability are limited, hindering the ability to characterize its sensitivity to environmental conditions. Here, we present a zonal section of POC fluxes at high vertical and spatial resolution from the GEOTRACES GP16 transect in the eastern tropical South Pacific, based on normalization to the radiogenic thorium isotope (230)Th. We find shallower carbon regeneration length scales than previous estimates for the oligotrophic South Pacific gyre, indicating less efficient carbon transfer to the deep ocean. Carbon regeneration is strongly inhibited within suboxic waters near the Peru coast. Canonical Martin curve power laws inadequately capture POC flux profiles at suboxic stations. We instead fit these profiles using an exponential function with flux preserved at depth, finding shallow regeneration but high POC sequestration below 1,000 m. Both regeneration length scales and POC flux at depth closely track the depths at which oxygen concentrations approach zero. Our findings imply that climate warming will result in reduced ocean carbon storage due to expanding oligotrophic gyres, but opposing effects on ocean carbon storage from expanding suboxic waters will require modeling and future work to disentangle.

25 pages, 11 figures, supporting information https://doi.org/10.1029/2020GB006769.-- Data Availability Statement: The new and compiled data reported here are available at the NOAA Paleoclimatology database (https://www.ncdc.noaa.gov/paleo/study/30512), Quantitative knowledge about the burial of sedimentary components at the seafloor has wide‐ranging implications in ocean science, from global climate to continental weathering. The use of 230Th‐normalized fluxes reduces uncertainties that many prior studies faced by accounting for the effects of sediment redistribution by bottom currents and minimizing the impact of age model uncertainty. Here we employ a recently compiled global data set of 230Th‐normalized fluxes with an updated database of seafloor surface sediment composition to derive atlases of the deep‐sea burial flux of calcium carbonate, biogenic opal, total organic carbon (TOC), nonbiogenic material, iron, mercury, and excess barium (Baxs). The spatial patterns of major component burial are mainly consistent with prior work, but the new quantitative estimates allow evaluations of deep‐sea budgets. Our integrated deep‐sea burial fluxes are 136 Tg C/yr CaCO3, 153 Tg Si/yr opal, 20Tg C/yr TOC, 220 Mg Hg/yr, and 2.6 Tg Baxs/yr. This opal flux is roughly a factor of 2 increase over previous estimates, with important implications for the global Si cycle. Sedimentary Fe fluxes reflect a mixture of sources including lithogenic material, hydrothermal inputs and authigenic phases. The fluxes of some commonly used paleo‐productivity proxies (TOC, biogenic opal, and Baxs) are not well‐correlated geographically with satellite‐based productivity estimates. Our new compilation of sedimentary fluxes provides detailed regional and global information, which will help refine the understanding of sediment preservation, This study was supported by the Past Global Changes (PAGES) project, which in turn received support from the Swiss Academy of Sciences and the US‐NSF. The work grew out of a 2018 workshop in Aix‐Marseille, France, funded by PAGES, GEOTRACES, SCOR, US‐NSF, Aix Marseille Université, and John Cantle Scientific, and the authors would like to acknowledge all attendees of this meeting. [...]. Christopher T. Hayes acknowledges support from US‐NSF awards 1658445 and 1737023. Some data compilation on Arctic shelf seas was supported by the Russian Science Foundation, grant number 20‐17‐00157. This work was also supported through project CRESCENDO (grant no. 641816, European Commission). Zanna Chase acknowledges support from the Australian Research Council’s Discovery Projects funding scheme (project DP180102357). Christopher R. German acknowledges US‐NSF awards 1235248 and 1234827, With the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI)

The Southern Ocean hosts complex connections between ocean physics, chemistry, and biology. Changes in these connections are hypothesized to be responsible for significant alterations of ocean biogeochemistry and carbon storage both on glacial‐interglacial timescales, and in the future due to anthropogenic forcing. Isotopes of thorium (²³⁰Th, ²³²Th) and protactinium (²³¹Pa) have been widely applied as tools to study paleoceanographic conditions in the Southern Ocean. However, understanding of the chemical behavior of these isotopes in the modern Southern Ocean has been limited by a paucity of high‐resolution observations. In this study, we present measurements of dissolved ²³⁰Th, ²³¹Pa, and ²³²Th on a meridional transect along 170°W from 67°S to 54°S in the Pacific sector of the Southern Ocean, with high vertical and meridional sampling resolution. We find Th/Pa fractionation factors below 1, highlighting the preferential removal of Pa relative to Th in a region with low lithogenic inputs where the particle flux is dominated by biogenic opal. We also find steep gradients in all three of these isotopes along neutral density surfaces from north to south, demonstrating the importance of isopycnal mixing in transporting these nuclides to the Southern Ocean. Our results suggest that ²³¹Pa and ²³⁰Th in the Southern Ocean are highly sensitive tracers of physical transport that may find use in studies of Southern Ocean biogeochemical‐physical connections in past, present, and future.

International audience; 230 Th normalization is a valuable paleoceanographic tool for reconstructing high-resolution sediment fluxes during the late Pleistocene (last~500,000 years). As its application has expanded to ever more diverse marine environments, the nuances of 230 Th systematics, with regard to particle type, particle size, lateral advective/diffusive redistribution, and other processes, have emerged. We synthesized over 1000 sedimentary records of 230 Th from across the global ocean at two time slices, the late Holocene (0-5,000 years ago, or 0-5 ka) and the Last Glacial Maximum (18.5-23.5 ka), and investigated the spatial structure of 230 Th-normalized mass fluxes. On a global scale, sedimentary mass fluxes were significantly higher during the Last Glacial Maximum (1.79-2.17 g/cm 2 kyr, 95% confidence) relative to the Holocene (1.48-1.68 g/cm 2 kyr, 95% confidence). We then examined the potential confounding influences of boundary scavenging, nepheloid layers, hydrothermal scavenging, size-dependent sediment fractionation, and carbonate dissolution on the efficacy of 230 Th as a constant flux proxy. Anomalous 230 Th behavior is sometimes observed proximal to hydrothermal ridges and in continental margins where high particle fluxes and steep continental slopes can lead to the combined effects of boundary scavenging and nepheloid interference. Notwithstanding these limitations, we found that 230Th normalization is a robust tool for determining sediment mass accumulation rates in the majority of pelagic marine settings (>1,000 m water depth).

Hydrothermal circulation and subsequent eruption of seawater at mid-ocean ridges and back-arc basins has great potential to modulate deep ocean biogeochemistry, acting as both a source and a sink for many trace elements and their isotopes. The influence of hydrothermal vents as a source of iron and manganese has been demonstrated in all ocean basins. However, the long-range impact of scavenging by hydrothermal particles has yet to be documented in detail. We use dissolved and particulate measurements of long-lived radiogenic ( 230 Th, 231 Pa) and primordial ( 232 Th) radionuclides to investigate the nature and geographic scale of scavenging processes occurring within a hydrothermal plume in the Southeast Pacific Ocean sampled during the GEOTRACES GP16 section. Due to their radioactive disequilibrium with respect to production by their parent uranium isotopes, 230 Th and 231 Pa provide unique insights into the rates of scavenging. We find strong deficits in 230 Th and 231 Pa coincident with elevated particulate Mn and Fe(OH) 3 , indicating that trace metal scavenging is widespread and likely the result of the strong affinity of trace metals for nanoparticulate metal surface sites. The chemical composition of the particulate material is closely linked to the scavenging intensity of 230 Th and 231 Pa, expressed as distribution coefficients between solid and solution. A comparison of 230 Th and 231 Pa inventories with mantle-derived 3 He as well as a mass balance of 230 Th and 231 Pa suggests continuous scavenging removal over the course of the entire 4000 km transect. Unlike the two radiogenic isotopes, 232 Th is enriched above what would be expected from assuming identical scavenging behavior to 230 Th, indicating a hydrothermal source of colloidal, unreactive 232 Th.

Thorium-230 (^(230)Th) is a fundamental tool for estimating sediment fluxes in the open ocean. Because ^(230)Th is rapidly scavenged by particles falling through the water column, the flux of ^(230)Th to underlying sediments is typically equal to its water column production rate. However, recent surveys suggest hydrothermal plumes are unusually efficient scavengers of ^(230)Th. Here we show that hydrothermal scavenging on the Southern East Pacific Rise (SEPR) resulted in ^(230)Th fluxes several times higher than the water column production rate during the last deglaciation. Elevated fluxes likely require diffusive transport of dissolved ^(230)Th from the ridge flanks towards the ridge crest. Depending on the length-scale of ^(230)Th transport, the resulting deficits in ^(230)Th may yield overestimates of sediment flux to ridge flank sediments. We also show that Fe fluxes at 19°S on the SEPR lag those at 11°S and 6°S by several thousand years, inconsistent with a signal driven by changes in deep water pH and oxygen levels. Instead, variable hydrothermal activity is the simplest explanation of the observed signals in the Pacific, Indian, and Atlantic basins.

Reconstructions of past changes in dissolved oxygen concentrations in the abyssal ocean are of interest to paleoceanographers because of their potential to help characterize and quantify the transfer of carbon between the atmosphere and the deep ocean. This potential, derived from the stoichiometric relationship between oxygen consumption and the regeneration of organic matter, has recently been expanded by compilations of core top observations for two proxies: the δ13C gradient between coeval infaunal and epifaunal benthic foraminifera (Δδ13C), and biomarker preservation. Here, we review these newer proxies, and the more established redox proxy authigenic uranium (aU), to critically evaluate our understanding of the controls on proxy signal production and preservation. We locate our work in the equatorial Pacific, presenting both new data and a compilation of existing records from thirty-two sediment cores to draw semi-quantitative conclusions about bottom water oxygen and respired carbon concentrations over the last glacial period. We find that the biogeochemical limitations on these proxies may be more substantial than previously appreciated, and therefore suggest several best-practice recommendations for their application. Despite the recognized data limitations, the compilation identifies the glacial Pacific Ocean as a dominant sink for CO2 at all depths below the modern oxygen minimum zone. Our review emphasizes the importance of multiproxy reconstructions, informed by site-specific records of paleoproductivity, in drawing coherent, internally consistent conclusions about glacial ocean oxygenation and carbon storage.