Paleo-oceanographic and -climatic reconstruction in the Southwest Pacific [ODP Site 1123] during MIS 11
Marine Isotope Stage 11 [424 to 374 ka] is unique compared to most other recent Quaternary interglacial periods due to its duration and orbital geometry, both of which have previously been cited as evidence that MIS 11 may be a suitable analogue to project future climate. This study aims to evaluate this prolonged warm period at a key site in the sparsely studied Southwest Pacific Ocean at Ocean Drilling Program [ODP] 1123. This cored site, situated at 3290 m water depth on the northern flank of the Chatham Rise, straddles the northern limit of the modern Subtropical Front, 1100 km east of New Zealand, where sediments record strong subtropical and subpolar signals over interglacial to glacial cycles. Two species of planktonic foraminifera were analysed, Globigerinoides ruber and Globigerina bulloides [Gs. ruber and Gg. bulloides], for trace elements and size-normalised test weights [SNW; Gg. bulloides only] in order to reconstruct ocean temperature, chemistry, structure and circulation during MIS 11. Gg. bulloides was found to have anomalously low SNW [~50% compared to modern specimens] implying either [i] poor calcification environment due to low CO₃⁻² concentrations, or [ii] post-mortem alteration either in the deep water column or ocean floor environment. Traditional dissolution proxies for ODP 1123 do not indicate significant dissolution during MIS 11. Nevertheless, the inception of modern carbonate platforms and reefs at this time leads to the hypothesis that CO₃⁻² concentrations in the surface ocean were low due to a shifting in the locus of carbonate production, and this is a potential cause, amongst other possibilities, of the low SNW in Gg. bulloides. However, calcification in a low CO₃⁻² concentration ocean does not appear to have significantly affected the geochemical proxies utilised in this study [Mg/Ca-derived paleo-ocean temperatures, δ¹⁸O and micro-nutrients Mn/Ca and Zn/Ca ratios as water-mass tracers] based on comparison with a similar study on younger sediments in the same core. The temperature difference between Gs. ruber and Gg. bulloides is the same as the modern temperature difference at ODP 1123, implying that Gs.ruber was also not markedly affected by either low CO₃⁻² concentrations during calcification or post-mortem dissolution. Laser ablation inductively coupled plasma mass spectrometry is utalised to measure in situ trace element ratios [Mg, Al, Ca, Mn, Zn and Sr/Ca], and reconstruct the thermal structure of the ocean’s upper 200 m. The main findings are [i] a well stratified upper ocean in warm periods punctuated by well mixed waters in cooler and presumably windier conditions; [ii] an invigorated South Pacific Gyre during the prolonged MIS 11 interglacial, resulting in a greater inflow of subtropical water to ODP 1123 as evinced by Mn/Ca and Zn/Ca ratios and supported by elevated subtropical foramiferal assemblages; [iii] paleo-ocean temperatures that indicate the mean MIS 11 sea surface temperature optimum was ca. 2°C warmer than present; and [iv] a spike in productivity is identified by elevated Mn/Ca and Zn/Ca ratios at ca. 400 ka, coinciding with a spike in eutrophic species abundance, indicating a period of significantly enhanced subtropical water influence. Records from other New Zealand sites reveal MIS 11 as a prolonged [up to 40 kyr] interglacial period, following a rapid and pronounced 10°C warming from the MIS 12 glacial. Deglaciation occurred 13 kyr earlier than the global benthic record. This rise was punctuated by an Antarctic Cold Reversal-like cooling confirming episodic sub-polar influences at the site. Some differences between the orbital configurations of MIS 1 and 11, particularly at the precessional scale, coupled with apparently unusual ocean chemistry [e.g., low CO₃⁻²] during MIS 11, suggest that MIS 11 may not be an ideal analogue for the Holocene.