Southwest Pacific Ocean response to a warming world: Using Mg/Ca, Zn/Ca, and Mn/Ca in foraminifera to track surface ocean water masses during the last deglaciation

Abstract

In situ measurements of Mg/Ca, Zn/Ca, Mn/Ca, and Ba/Ca in Globigerinoides bulloides and Globigerina ruber from southwest Pacific core top sites and plankton tow are reported and their potential as paleoproxies is explored. The modern samples cover 20° of latitude from 34°S to 54°S, 7–19°C water temperature, and variable influence of subantarctic (SAW) and subtropical (STW) surface waters. Trace element signatures recorded in core top and plankton tow planktic foraminifera are examined in the context of the chemistry and nutrient profiles of their modern water masses. Our observations suggest that Zn/Ca and Mn/Ca may have the potential to trace SAW and STW. Intraspecies and interspecies offsets identified by in situ measurements of Mg/Ca and Zn/Ca indicate that these ratios may also record changes in thermal and nutrient stratification in the upper ocean. We apply these potential proxies to fossilized foraminifera from the high‐resolution core MD97 2121. At the Last Glacial Maximum, surface water Mg/Ca temperature estimates indicate that temperatures were approximately 6–7°C lower than those of the present, accompanied by low levels of Mn/Ca and Zn/Ca and minimal thermal and nutrient stratification. This is consistent with regional dominance of SAW and reduced STW inflow associated with a reduced South Pacific Gyre (SPG). Upper ocean thermal and nutrient stratification collapsed during the Antarctic Cold Reversal, before poleward migration of the zonal winds and ocean fronts invigorated the SPG and increased STW inflow in the early Holocene. Together with reduced winds, this favored a stratified upper ocean from circa 10 ka to the present.

Southwest Pacific Ocean surface reservoir ages since the last glaciation: Circulation insights from multiple‐core studies

“…These competing factors counteract one another in generating a reservoir age, but the combination appears to have served to both make the reservoir age older and to maintain the enhanced gradient in surface ages between subtropical and subpolar surface water masses in the glaciation. The fact that the Hikurangi Trough appears to have been more influenced by subantarctic water during the glaciation and early deglaciation than the Bay of Plenty [ Carter et al ., ; Marr et al ., ; Schiraldi et al ., ] supports this scenario.…”

Ba incorporation in benthic foraminifera

“…So far, Ba / Ca values have been reported for planktonic (Boyle, 1981;Lea and Boyle, 1991;Lea and Spero, 1992;1994;Hönisch et al, 2011;Marr et al, 2013;Hoffmann et al, 2014) and low-Mg benthic species (Lea, 1995;, 1993Reichart et al, 2003). Although Mg / Ca is known to vary greatly between (benthic) foraminiferal species (between ∼ 1 and ∼ 150 mmol mol −1 ; Toyofuku et al, 2000;Bentov and Erez, 2006;Wit et al, 2012) Ba / Ca ratios, which is only rarely investigated in species producing high-Mg calcite (Evans et al, 2015;Van Dijk et al, 2017).…”

Modern, Preindustrial, and Past (Last 25 ka) Carbon Isotopic (δ13C) Variability in the Surface Waters of the Southwest Pacific

“…In the NCR the δ 13 C G . bulloides TC is higher than the late Holocene, although still within uncertainty (Figure ), indicative of slightly higher biological productivity, which may have been the result of SASW leakage around either end of Chatham Rise contributing more nutrients to this region (Crundwell et al, ; Marr et al, ; Nelson et al, ). This is supported by high carbonate mass accumulation rates in core MD97‐2121 during the glacial (Carter et al, ).…”