Trace element composition of suspended particulate matter along three meridional CLIVAR sections in the Indian and Southern Oceans: Impact of scavenging on Al distributions

Publication Date

June 2018

Document Type

Article

Publication Title

Chemical Geology

DOI

10.1016/j.chemgeo.2018.06.015

Abstract

We present the trace element composition (Al, Si, P, Ca, Fe, and Zn) of suspended particulate matter (SPM) samples collected in the upper 1000 m along U.S. CLIVAR/CO2 Repeat Hydrography meridional section I09N/I08S from the Southern Ocean to the Bay of Bengal at 95°E and in surface waters along meridional section I06S from the South African margin to the Antarctic shelf edge at 30°E. A band of elevated surface-ocean pCa (0.5–0.3 μM) over 30–60°S is consistent with the global pattern of high coccolithophore production encircling the Southern Ocean suggested by remote sensing. Over 50–60°S, both sections exhibit a sharp transition from Ca-rich to Si-rich particulate matter (up to 2.8–6.5 μM), indicating a change from coccolithophore to diatom communities in waters with low residual nitrate. Along I08S, diatom blooms display elevated pZn concentrations and Zn:P ratios (14.9 + 5.7 mmol mol−1), reflecting cellular Zn stoichiometry of Southern Ocean diatoms. Regions of high primary productivity in the Southern Ocean are coincident with unexpectedly high surface-ocean pAl concentrations (max 12.6–26.5 nM) compared to background concentrations (typically <2 nM along I09N/I08S and <5 nM along I06S). We estimate that >50% of surface-ocean pAl in these regions originates from adsorptive scavenging of dissolved Al onto biogenic particles. Along I08S, dissolved Al reservoirs likely originate from transport of Al-enriched waters by the Agulhas Return Current, local dust deposition, and seasonal ice melt. Along I06S, pAl distributions suggest larger inputs of dust to the surface mixed layer than have previously been described. By contrast, pFe concentrations remain low in both surface (<0.3 nM I09N/I08S, 0.1–0.8 nM I06S) and intermediate (<0.4 nM I09N/I08S) waters away from coastal and shelf inputs. Inputs of lithogenic material from resuspended shelf sediments are apparent in the distributions of pFe and pAl close to the Antarctic margin (up to 10 nM pFe and 23.4 nM pAl) and the South African coast (up to 3.1 nM pFe and 8 nM pAl). At intermediate depths along I08S, elevated pFe concentrations (up to 1.0 nM) are observed downstream of the Kerguelen Plateau, reflecting lateral transport of resuspended plateau sediments or glacial runoff.This article is part of a special issue entitled: Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond.

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