The competition between coastal trace metal fluxes and oceanic mixing from the ¹⁰Be/⁹Be ratio: Implications for sedimentary records

Abstract

At an ocean margin site 37°S offshore Chile, we use the meteoric cosmogenic 10Be/9Be ratio to trace changes in terrestrial particulate composition due to exchange with seawater. We analyzed the marine authigenic phase in surface sediments along a coast‐perpendicular transect and compared to samples from their riverine source. We find evidence for growth of authigenic rims through coprecipitation, not via reversible adsorption, that incorporate an open ocean 10Be/9Be signature from a deep water source only 30 km from the coast, overprinting terrestrial 10Be/9Be signatures. Together with increasing 10Be/9Be ratios, particulate‐bound Fe concentrations increase, which we attribute to release of Fe‐rich pore waters during boundary exchange in the sediment. The implications for the use of 10Be/9Be in sedimentary records for paleodenudation flux reconstructions are the following: in coast‐proximal sites the authigenic record will likely preserve local riverine ratios unaffected by exchange with seawater, whereas sites beneath well‐mixed seawater will preserve global flux signatures.

10Be/9Be Ratios Reveal Marine Authigenic Clay Formation

“…It is likely that the clay‐sized fraction retrieves its additional 10 Be from that same seawater source as the amorphous hydroxide (am‐ox) fraction previously measured by Wittmann et al (). The am‐ox fraction shows a tenfold increase in [ 10 Be] and 10 Be/ 9 Be ratio from river to ocean sediment (Figure ), whereas the increase in the clay‐sized fraction is fourfold.…”

“…This increase, which is due to high meteoric 10 Be concentrations in seawater, requires the formation of a new silicate phase in marine surface sediment. Results from a previous study using the same sample set similarly suggest that 10 Be/ 9 Be ratios of marine authigenic iron‐oxy‐hydroxide phases comprise a mixture of a terrestrial and an oceanic component (Wittmann et al, ). In both cases, the seawater‐derived 10 Be is incorporated into marine authigenic phases in which dissolved Be features a 10 Be(meteoric)/ 9 Be ratio that is almost two orders of magnitude higher than the ratio of the terrigenous riverine source.…”

“…In this study we test whether the clay‐sized sediment fraction contains Be that either preserves a 10 Be/ 9 Be ratio representing terrestrial erosion or records marine authigenic clay formation. We show that such pattern of 10 Be/ 9 Be increase (Wittmann et al, ) also applies to the clay‐sized component of river and offshore sediments and explore the consequences of this finding for the use of the 10 Be/ 9 Be proxy as a tracer of authigenic clay formation in the ocean.…”

“…This rapid increase in [ 10 Be] reac was interpreted as being related to the growth of authigenic rims on substrate particles through coprecipitation incorporating Be with a high open-ocean 10 Be/ 9 Be signature only 30 km from the coast. Thus, the inner authigenic rim preserved the 10 Be/ 9 Be characteristic of continental erosion, whereas the outer rim incorporates Be with an open-ocean ( 10 Be/ 9 Be) reac ratio (Wittmann et al, 2017). In this study we test whether the clay-sized sediment fraction contains Be that either preserves a 10 Be/ 9 Be ratio representing terrestrial erosion or records marine authigenic clay formation.…”

10Be/9Be Ratios Reflect Antarctic Ice Sheet Freshwater Discharge During Pliocene Warming

“…We targeted a marine sedimentary record in which EAIS fluctuations are both distinct and well documented. It was important to sample sediments proximal to the ice sheet to document ice sheet/terrestrial contributions before that signal was overprinted after exchange with well‐mixed water masses, which instead reflect a deep marine signal (Wittmann et al, ). Indeed, while global, deep marine 10 Be/ 9 Be records fluctuate negligibly throughout glacial‐interglacial transitions (von Blanckenburg et al, ; Willenbring & von Blanckenburg, ), records closer to glaciated margins taken at finer sampling resolutions indicate a sensitive response in 10 Be/ 9 Be to ice mass changes (Eisenhauer et al, ; Simon et al, ).…”