Abstract
Abstract230Th 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 of230Th systematics, with regard to particle type, particle size, lateral advective/diffusive redistribution, and other processes, have emerged. We synthesized over 1000 sedimentary records of230Th 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 of230Th‐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 of230Th as a constant flux proxy. Anomalous230Th 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 that230Th normalization is a robust tool for determining sediment mass accumulation rates in the majority of pelagic marine settings (>1,000 m water depth).
Highlights
Burial fluxes of different components of marine sediment provide insight into a wide variety of surface processes that are central to the Earth system, including marine export productivity, windblown dust deposition on the sea surface, carbon storage as organic matter and calcium carbonate, and hydrothermal activity on the seafloor
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
Constant flux proxies are geochemical parameters with well‐constrained and stable source functions, such as 230Th (Bacon, 1984; Francois et al, 2004) and 3He (Marcantonio et al, 1996; McGee & Mukhopadhyay, 2013; Schlosser & Winckler, 2002; Winckler et al, 2004). 230Th is produced by the steady decay of uranium dissolved in seawater, after which it is rapidly removed by sinking particles and buried on the seafloor (Bacon, 1984; Francois et al, 1990; Francois et al, 2004; Suman & Bacon, 1989)
Summary
Burial fluxes of different components of marine sediment provide insight into a wide variety of surface processes that are central to the Earth system, including marine export productivity, windblown dust deposition on the sea surface, carbon storage as organic matter and calcium carbonate, and hydrothermal activity on the seafloor. The temporal resolution of this approach is limited by the robustness of the age model, including the number of chronological tie points and their associated errors (e.g., Francois et al, 2004) This approach can be biased by sediment redistribution on the seafloor (e.g., Johnson & Johnson, 1970), where lateral sediment transport can exceed the vertical rain of particles from the water column. The GEOTRACES program and associated modeling studies have improved our understanding of 230Th cycling in the modern ocean With these changes in mind, and the increasing utilization of sedimentary 230Th across the global ocean, we have produced an updated compilation that provides an overview of the methodology and current understanding of the 230Th normalization technique on a global scale
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