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

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Costa, K. M., Hayes, C. T., Anderson, R. F., Pavia, F. J., Bausch, A., Deng, F., Dutay, J., Geibert, W., Heinze, C., Henderson, G., Hillaire-Marcel, C., Hoffmann, S., Jaccard, S. L., Jacobel, A. W., Kienast, S. S., Kipp, L., Lerner, P., Lippold, J., Lund, D., Marcantonio, F., McGee, D., McManus, J. F., Mekik, F., Middleton, J. L., Missiaen, L., Not, C., Pichat, S., Robinson, L. F., Rowland, G. H., Roy-Barman, M., Alessandro, Torfstein, A., Winckler, G., & Zhou, Y. 230 Th normalization: new insights on an essential tool for quantifying sedimentary fluxes in the modern and quaternary ocean. Paleoceanography and Paleoclimatology, 35(2), (2020): e2019PA003820, doi:10.1029/2019PA003820.230Th 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 diverse marine environments, the nuances of 230Th systematics, with regard 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 (0–5,000 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 marine settings (>1,000 m water depth).We thank Zanna Chase and one anonymous reviewer for valuable feedback. K. M. C. was supported by a Postdoctoral Scholarship at WHOI. L. M. acknowledges funding from the Australian Research Council grant DP180100048. The contribution of C. T. H., J. F. M., and R. F. A. were supported in part by the U.S. National Science Foundation (US‐NSF). G. H. R. was supported by the Natural Environment Research Council (grant NE/L002434/1). S. L. J. acknowledges support from the Swiss National Science Foundation (grants PP002P2_144811 and PP00P2_172915). 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. This 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. All data are publicly available as supporting information to this document and on the National Center for Environmental Information (NCEI) at https://www.ncdc.noaa.gov/paleo/study/28791

\u3csup\u3e230\u3c/sup\u3eTh Normalization: New Insights on an Essential Tool for Quantifying Sedimentary Fluxes in the Modern and Quaternary Ocean

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 diverse marine environments, the nuances of 230Th systematics, with regard 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 (0–5,000 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 marine settings (\u3e1,000 m water depth)

Radiogenic Nd isotope signatures of marine sediments (bulk and detrital), foraminifera and dolostone grains from sediment core MD99-2227

This dataset includes the results of the analysis of a marine sediment downcore record recovered from the Labrador Sea: core MD99-2227 (58° 12.38 N, 48° 22.22 W, 3460 m). Analysis includes measurement of the radiogenic Nd isotope signatures in the authigenic Fe-Mn oxyhydroxide fraction of the bulk sediment (leachates) and of the totally dissolved residual silicate (detrital) as well as analysis of uncleaned foraminifera. The downcore records were sampled with resolution of about 1 kyr and cover roughly the last 33 ka

Radiogenic Nd isotope signatures of progressively leached dolostone grains from 4 sediment cores in the Labrador Sea

This dataset includes the results of the analysis of 4 marine sediment downcore records recovered from the Labrador Sea: core MD99-2227 (58° 12.38 N, 48° 22.22 W, 3460 m); core HU08-029-004 (61°27.49 N, 58°02.11 W, 2674 m); HU84-030-021 (58°22.06 N, 57°30.42 W, 2853 m); core HU91-045-094,(50°12.26 N, 45°41.14 W, 3448 m). Analysis includes measurement of the radiogenic Nd isotope signatures progressively leached dolostone grains, picked from wet-sieved < 63µm sediment fraction of marine sediment downcores. The downcore records were sampled with resolution of about 1 kyr and cover roughly the last 33 ka

Authigenic and detrital carbonate Nd isotope records reflect pulses of detrital material input to the Labrador Sea during the Heinrich Stadials

Limited constraints on the variability of the deep-water production in the Labrador Sea complicate reconstructions of the strength of the Atlantic Meridional Overturning Circulation (AMOC) during the Late Quaternary. Large volumes of detrital carbonates were repeatedly deposited in the Labrador Sea during the last 32 kyr, potentially affecting radiogenic Nd isotope signatures. To investigate this the Nd isotope compositions of deep and intermediate waters were extracted from the authigenic Fe-Mn oxyhydroxide fraction, foraminiferal coatings, the residual silicates and leachates of dolostone grains. We provide a first order estimation of Nd release via dissolution of detrital carbonates and its contribution to the authigenic ԑNd signatures in the Labrador Sea. During the Last Glacial Maximum the Nd isotope signatures in the Labrador Sea would allow active water mass mixing with more radiogenic ɛNd values (-12.6 and -14) prevailing in its eastern part whereas less radiogenic values (ɛNd ∼ -18.4) were found on the western Labrador slope. The deposition of detrital carbonates during Heinrich stadials (2,1) was accompanied by negative detrital and authigenic Nd isotope excursions (ɛNd ∼ -31) that were likely controlled by dissolution of dolostone or dolostone associated mineral inclusions. This highly unradiogenic signal dominated the authigenic phases and individual water masses in the Labrador Sea, serving as potential source of highly unradiogenic Nd to the North Atlantic region, while exported southward. The Holocene authigenic ɛNd signatures of the coatings and leachates significantly differed from those of the detrital silicates, approaching modern bottom water mass signatures during the Late Holocene. Key Points - Estimation of Nd release via dissolution of detrital carbonates and its contribution to the authigenic ԑNd signatures in the Labrador Sea - Dissolution of detrital dolostones in the water column during Heinrich stadials at least partially controlled ɛNd signatures - During the LGM generally more radiogenic signatures possibly indicate active water mass advection and mixing in the Labrador Se

Radiogenic Nd isotope signatures and REE concentrations of marine sediments (bulk and detrital), foraminifera and dolostone grains from 4 sediment downcore records from the Labrador Sea

This dataset includes the results of the analysis of 4 marine sediment downcore records recovered from the Labrador Sea: core MD99-2227 (58° 12.38 N, 48° 22.22 W, 3460 m); core HU08-029-004 (61°27.49 N, 58°02.11 W, 2674 m); HU84-030-021 (58°22.06 N, 57°30.42 W, 2853 m); core HU91-045-094,(50°12.26 N, 45°41.14 W, 3448 m) as well as a set of surface sediment samples. Analysis includes measurement of the radiogenic Nd isotope signatures in the authigenic Fe-Mn oxyhydroxide fraction of the bulk sediment (leachates) and of the totally dissolved residual silicate (detrital) as well as analysis of uncleaned foraminifera. Data complemented by measurements of the Rare Earth Element concentrations, some major elements (Fe, Mg, Ca, Al) and radiogenic Nd isotope signatures of the dolostone grains, picked from wet-sieved < 63µm sediment fraction of marine sediment downcores. The downcore records were sampled with resolution of about 1 kyr and cover roughly the last 33 ka

Radiogenic Nd isotope signatures of marine sediments (bulk and detrital) and dolostone grains from surface sediments in the Labrador Sea

This dataset includes the results of the analysis of a set of surface sediment samples recovered from the Labrador Sea. Analysis includes measurement of the radiogenic Nd isotope signatures in the authigenic Fe-Mn oxyhydroxide fraction of the bulk sediment (leachates) and of the totally dissolved residual silicate (detrital) as well as analysis of uncleaned foraminifera

Radiogenic Nd isotope signatures of marine sediments (bulk and detrital), foraminifera and dolostone grains from sediment core HU08-029-0004

This dataset includes the results of the analysis of a marine sediment downcore record recovered from the Labrador Sea: core HU08-029-004 (61°27.49 N, 58°02.11 W, 2674 m). Analysis includes measurement of the radiogenic Nd isotope signatures in the authigenic Fe-Mn oxyhydroxide fraction of the bulk sediment (leachates) and of the totally dissolved residual silicate (detrital) as well as analysis of uncleaned foraminifera. The downcore records were sampled with resolution of about 1 kyr and cover roughly the last 33 ka

Radiogenic Nd isotope signatures of marine sediments (bulk and detrital), foraminifera and dolostone grains from sediment core HU91-045-094

This dataset includes the results of the analysis of a marine sediment downcore record recovered from the Labrador Sea: core HU91-045-094 (50°12.26 N, 45°41.14 W, 3448 m). Analysis includes measurement of the radiogenic Nd isotope signatures in the authigenic Fe-Mn oxyhydroxide fraction of the bulk sediment (leachates) and of the totally dissolved residual silicate (detrital) as well as analysis of uncleaned foraminifera. The downcore records were sampled with resolution of about 1 kyr and cover roughly the last 33 ka