Comparing glacial and Holocene opal fluxes in the Pacific sector of the Southern Ocean

Abstract

[1] The silicic acid leakage hypothesis (SALH) predicts that during glacial periods excess silicic acid was transported from the Southern Ocean to lower latitudes, which favored diatom production over coccolithophorid production and caused a drawdown of atmospheric CO 2 . Downcore records of 230 Th-normalized opal (biogenic silica) fluxes from 31 cores in the Pacific sector of the Southern Ocean were used to compare diatom productivity during the last glacial period to that of the Holocene and to examine the evidence for increased glacial Si export to the tropics. Average glacial opal fluxes south of the modern Antarctic Polar Front (APF) were less than during the Holocene, while average glacial opal fluxes north of the APF were greater than during the Holocene. However, the magnitude of the increase north of the APF was not enough to offset decreased fluxes to the south, resulting in a decrease in opal burial in the Pacific sector of the Southern Ocean during the last glacial period, equivalent to approximately 15 Gt opal ka À1 . This is consistent with the work of Chase et al. (2003a), and satisfies the primary requirement of the SALH, assuming that the upwelled supply of Si was approximately equivalent during the Holocene and the glacial period. However, previous results from the equatorial oceans are inconsistent with the other predictions of the SALH, namely that either the C org :CaCO 3 ratio or the rate of opal burial should have increased during glacial periods. We compare the magnitudes of changes in the Southern Ocean and the tropics and suggest that Si escaping the glacial Southern Ocean must have had an alternate destination, possibly the continental margins. There is currently insufficient data to test this hypothesis, but the existence of this sink and its potential impact on glacial pCO 2 remain interesting topics for future study.

The simulated climate of the Last Glacial Maximum and insights into the global marine carbon cycle

“…The weakened efficiency of our simulated biological pump can be attributed, in part, to a large decrease in export production from the subantarctic zone. This feature is in direct conflict with palaeoproductivity proxies in the Atlantic and Indian sectors of the subantarctic Ocean (Anderson et al, 2002Chase et al, 2001;Jaccard et al, 2013;Nürnberg et al, 1997) and some parts of the Pacific sector (Bradtmiller et al, 2009;Lamy et al, 2014). Outside of the Southern Ocean, the reduction in export production in the O-LGM experiment is largely consistent with palaeoproductivity evidence (see Introduction).…”

“…In the Southern Ocean, the Atlantic and Indian sectors of the subantarctic zone experienced a greater flux of organics (Anderson et al, 2002Chase et al, 2001;Jaccard et al, 2013;Nürnberg et al, 1997). Whether this was also the case for the Pacific sector remains under debate, with some evidence for increase (Bradtmiller et al, 2009;Lamy et al, 2014) and some for no change (Bostock et al, 2013;Chase et al, 2003). Meanwhile, it is widely accepted that waters south of the Antarctic Polar Front were reduced in their productivity (Bostock et al, 2013;Chase et al, 2003;Elderfield and Rickaby, 2000;Francois et al, 1997;Frank et al, 2000;Kohfeld et al, 2005;Kumar et al, 1995;Mortlock et al, 1991;Ninnemann and Charles, 1997;Shemesh et al, 1993), likely due to increased sea ice extent (Benz et al, 2016;Gersonde et al, 2005;Jaccard et al, 2013) and stratification Jaccard et al, 2005).…”

Biosilicification Drives a Decline of Dissolved Si in the Oceans through Geologic Time

“…Changes in weathering processes, as a result of the waxing and waning of ice-sheets during glacial cycles, and climatically-driven changes in biological production could shift oceanic DSi over timescales equivalent to the residence time of Si in the ocean (∼10-15 ky; Georg et al, 2009;Frings et al, 2016). More regional changes in DSi distribution could be triggered on millennial to centennial timescales through changes in the configuration of oceanic circulation cells, wind-driven upwelling systems , and biological changes in the regions of deepwater formation (Brzezinski et al, 2002;Bradtmiller et al, 2009;Meckler et al, 2013;Hendry and Brzezinski, 2014). …”

A Review of the Stable Isotope Bio-geochemistry of the Global Silicon Cycle and Its Associated Trace Elements

“…In addition to using the relative bSiO 2 accumulation rates between the Southern Ocean and lower latitudes as a test for Si export (e.g., Bradtmiller et al, 2009;Meckler et al, 2013), it is possible to use combined geochemical proxy records to test the SALH (Rousseau et al, 2016). Biogenic SiO 2 archives point toward abrupt shifts in productivity and DSi utilization at glacial terminations, rather than changes on glacialinterglacial timescales (e.g., using coupled spicule, hand-picked radiolarian and diatoms from archives in the Sargasso Sea; Hendry and Brzezinski, 2014).…”