230Th Normalization: New Insights on an Essential Tool for Quantifying Sedimentary Fluxes in the Modern and Quaternary Ocean

Costa, Kassandra M; Hayes, Christopher T.; Anderson, Robert F.; Pavia, F. J.; Bausch, Alexandra R.; Deng, Feifei; Dutay, Jean-Claude; Geibert, Walter; Heinze, Christoph; Henderson, Gideon M.; Hillaire‐Marcel, Claude; Hoffmann, S. S.; Jaccard, Samuel L.; Jacobel, Allison W; Kienast, Markus; Kipp, Lauren; Lerner, Paul; Lippold, Jörg; Lund, David C; Marcantonio, Franco; McGee, David; McManus, Jerry F; Mekik, Figen; Middleton, Jennifer L.; Missiaen, Lise; Not, Christelle; Pichat, Sylvain; Robinson, Laura F.; Rowland, George H.; Roy‐Barman, Matthieu; Tagliabue, Alessandro; Torfstein, Adi; Winckler, Gisela; Zhou, Yuxin

doi:10.1029/2019pa003820

Cited by 71 publications

(73 citation statements)

References 268 publications

(438 reference statements)

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“…where 0.2429 and 1.3297 are constants to convert 232 Th from concentration in μg/g to activity in dpm/g and U from activity in dpm/g to concentration in μg/g, respectively, and (U/Th) AR is an estimation of the U/Th activity ratio of detrital material. A range of estimations is available for (U/Th) AR between 0.4 and 0.8; a value of 0.5 was used here, which is consistent with most other prior U/Th studies in the Atlantic sector of the Southern Ocean (Costa et al, 2020). The mean ± standard deviation of all detrital uranium corrections for the whole core was 9.5 ± 8.3% of the measured uranium.…”

Section: Methodssupporting

confidence: 76%

Changes in Antarctic Bottom Water Formation During Interglacial Periods

Glasscock

Hayes

Redmond

et al. 2020

Paleoceanog and Paleoclimatol

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In the modern Southern Ocean and during the last interglacial period, Marine Isotope Stage 5e, there are observations that point to reduced Antarctic Bottom Water (AABW) formation. These reductions are believed to be driven by an increase in the strength of the Southern Ocean density stratification due to Antarctic ice melt‐induced surface water freshening. Any reduction in AABW formation has important implications for global climate as AABW plays a vital role in the cycling of carbon in the world's ocean. The primary question this study seeks to answer is do these AABW reductions occur during any of the other interglacials of the past 470,000 years? To study AABW changes in the paleoceanographic record, we look at changes in the redox record. Newly formed AABW is oxygen‐rich, so any reduction should lead to a decrease in oxygen concentrations in the deep Southern Ocean. The trace element uranium is useful for studying these redox changes as it is enriched in marine sediments under low‐oxygen conditions. When accounting for other factors, such as paleoproductivity, that can also decrease the oxygen concentrations in sedimentary porewater, it is possible to identify changes in AABW using authigenic uranium. The survey conducted by this study found a possible AABW reduction during late Marine Isotope Stage 11 (~397 ka). The cause of this event is less clear than others studied, and we explore the possibilities of ice melt‐induced freshening or a change in the position or strength of the Southern Hemisphere westerly winds.

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Section: Methodssupporting

confidence: 76%

Changes in Antarctic Bottom Water Formation During Interglacial Periods

Glasscock

Hayes

Redmond

et al. 2020

Paleoceanog and Paleoclimatol

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“…Modeling 230 Th in the ocean began with 1-D analytical models (Bacon & Anderson, 1982;Clegg et al, 1991;Nozaki et al, 1981;Nozaki & Horibe, 1983;Roy-Barman et al, 1996), which demonstrated that only reversible scavenging was able to reproduce the observations of both the dissolved and particulate vertical profiles. To explain complex (nonlinear) 230 Th profiles, more elaborate box models were developed that could account for different transport conditions and particle regimes under different ocean conditions, such as upwelling of deep water masses in the Southern Ocean (Chase et al, 2003;Roy-Barman et al, 2019;Rutgers van der Loeff et al, 2016;Rutgers van der Loeff & Berger, 1993;Venchiarutti et al, 2011), convection of deep water masses in the North Atlantic (Moran et al, 1995(Moran et al, , 1997Vogler et al, 1998), and lateral exchange between open ocean and ocean margin regimes (Anderson et al, 1983;Roy-Barman et al, 2009). More recently, 230 Th has also been integrated into complex geographic schemes in 2-D models (Luo et al, 2010;Marchal et al, 2000), in 3-D models of intermediate complexity (Henderson et al, 1999;Missiaen et al, 2019;Rempfer et al, 2017;Siddall et al, 2007Siddall et al, , 2005, and in global climate models (GCMs) (Dutay et al, 2009;Gu & Liu, 2017;Rogan et al, 2016;Van Hulten et al, 2018).…”

Section: Sediment Fluxes Under Lgm Climate Conditionsmentioning

confidence: 99%

230Th normalization: New insights on an essential tool for quantifying sedimentary fluxes in the modern and Quaternary ocean

Costa¹

2020

Preprint

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230Th-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 complex marine environments, the nuances of 230Th systematics, with regards to particle type, particle size, lateral advective/diffusive redistribution, and other processes, have emerged. We synthesized over 1000 sedimentary records of 230Th from across the global ocean at two time slices, the Late Holocene (0-5000 years ago, or 0-5 ka) and the Last Glacial Maximum (18.5-23.5 ka), and investigated the spatial structure of 230Th-normalized mass fluxes. On a global scale, sedimentary mass fluxes were significantly higher during the Last Glacial Maximum (1.79-2.17 g/cm2kyr, 95% confidence) relative to the Holocene (1.48-1.68 g/cm2kyr, 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 230Th as a constant flux proxy. Anomalous 230Th 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 (> 1000 m) marine settings.

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“…This is in part because global models of dust input and iron dynamics still have significant uncertainties. Dust deposition models have few observational constraints over the open ocean, particularly in the South Pacific, given its remoteness from the continents and lack of well‐dated surface sediments (Albani et al, 2014; Costa et al, 2020; Kienast et al, 2016). Different global iron models vary over orders of magnitude in their simulated deposition rates from dust and in their average water column residence times of dissolved iron (Tagliabue et al, 2016).…”

Section: Introductionmentioning

confidence: 99%

Atmospheric Dust Inputs, Iron Cycling, and Biogeochemical Connections in the South Pacific Ocean From Thorium Isotopes

Pavia

Anderson

Winckler

et al. 2020

Global Biogeochemical Cycles

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One of the primary sources of micronutrients to the sea surface in remote ocean regions is the deposition of atmospheric dust. Geographic patterns in biogeochemical processes such as primary production and nitrogen fixation that require micronutrients like iron (Fe) are modulated in part by the spatial distribution of dust supply. Global models of dust deposition rates are poorly calibrated in the open ocean, owing to the difficulty of determining dust fluxes in sparsely sampled regions. We present new estimates of dust and Fe input rates from measurements of dissolved and particulate thorium isotopes 230 Th and 232 Th on the FS Sonne SO245 section (GEOTRACES process study GPpr09) in the South Pacific. We first discuss high-resolution upper water column profiles of Th isotopes and the implications for the systematics of dust flux reconstructions from seawater Th measurements. We find dust fluxes in the center of the highly oligotrophic South Pacific Gyre that are the lowest of any mean annual dust input rates measured in the global oceans, but that are 1-2 orders of magnitude higher than those estimated by global dust models. We also determine dust-borne Fe fluxes and reassess the importance of individual Fe sources to the surface South Pacific Gyre, finding that dust dissolution, not vertical or lateral diffusion, is the primary Fe source. Finally, we combine our estimates of Fe flux in dust with previously published cellular and enzymatic quotas to determine theoretical upper limits on annual average nitrogen fixation rates for a given Fe deposition rate.

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²³⁰Th Normalization: New Insights on an Essential Tool for Quantifying Sedimentary Fluxes in the Modern and Quaternary Ocean

Cited by 71 publications

References 268 publications

Changes in Antarctic Bottom Water Formation During Interglacial Periods

Changes in Antarctic Bottom Water Formation During Interglacial Periods

230Th normalization: New insights on an essential tool for quantifying sedimentary fluxes in the modern and Quaternary ocean

Atmospheric Dust Inputs, Iron Cycling, and Biogeochemical Connections in the South Pacific Ocean From Thorium Isotopes

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