NOAA/WDS Paleoclimatology - North Atlantic (DSDP 607) Geochemistry Data and BWT Reconstruction during the Mid-Pleistocene Transition

During the Mid-Pleistocene Transition (MPT), the dominant glacial-interglacial cyclicity as inferred from the marine d18O records of benthic foraminifera (d18Obenthic) changed from 41 kyr to 100 kyr years in the absence of a comparable change in orbital forcing. Currently, only two Mg/Ca-derived, high-resolution bottom water temperature (BWT) records exist that can be used with d18Obenthic records to separate temperature and ice volume signals over the Pleistocene. However, these two BWT records suggest a different pattern of climate change occurred over the MPTda record from North Atlantic DSDP Site 607 suggests BWT decreased with no long-term trend in ice volume over the MPT, while South Pacific ODP Site 1123 suggests that BWT has been relatively stable over the last 1.5 Myr but that there was an abrupt increase in ice volume at ~900 kyr. In this paper we attempt to reconcile these two views of climate change across the MPT. Specifically, we investigated the suggestion that the secular BWT trend obtained from Mg/Ca measurements on Cibicidoides wuellerstorfi and Oridorsalis umbonatus species from N. Atlantic Site 607 is biased by the possible influence of D[CO3 2_] on Mg/Ca values in these species by generating a low-resolution BWT record using Uvigerina spp., a genus whose Mg/Ca values are not thought to be influenced by D[CO3 2_]. We find a long-term BWT cooling of ~2e3_C occurred from 1500 to ~500 kyr in the N. Atlantic, consistent with the previously generated C. wuellerstorfi and O. umbonatus BWT record. We also find that changes in ocean circulation likely influenced d18Obenthic, BWT, and d18Oseawater records across the MPT. N. Atlantic BWT cooling starting at ~1.2 Ma, presumably driven by high-latitude cooling, may have been a necessary precursor to a threshold response in climate-ice sheet behavior at ~900 ka. At that point, a modest increase in ice volume and thermohaline reorganization may have caused enhanced sensitivity to the 100 kyr orbital cycle.

在中更新世转型期（Mid-Pleistocene Transition, MPT），基于底栖有孔虫（benthic foraminifera）海洋壳体δ¹⁸O记录（d18O_benthic）所推断的主导冰期-间冰期旋回周期，从41 kyr转变为100 kyr，而此时轨道强迫并未出现相应量级的变化。目前仅有两项基于Mg/Ca比值重建的高分辨率底层海水温度（bottom water temperature, BWT）记录，可与d18O_benthic记录配合，以分离更新世（Pleistocene）时期的温度与冰体体积信号。然而这两项BWT记录所揭示的气候变化模式存在差异：北大西洋深海钻探计划（Deep Sea Drilling Project, DSDP）607站位的MPT记录显示，BWT呈下降趋势且冰体体积无长期变化趋势；而南太平洋大洋钻探计划（Ocean Drilling Program, ODP）1123站位的记录则表明，过去150万年以来BWT相对稳定，但在约900 kyr时冰体体积出现突增。本文旨在调和这两种关于MPT时期气候变化的观点。具体而言，针对北大西洋607站位希氏双筛虫（Cibicidoides wuellerstorfi）和圆辐虫（Oridorsalis umbonatus）的Mg/Ca测量所得到的长期BWT趋势，可能受溶解态碳酸根离子（D[CO₃²⁻]）对该类群Mg/Ca比值的潜在影响而存在偏差。为此，我们利用织纹虫属（Uvigerina spp.）——其Mg/Ca比值被认为不受D[CO₃²⁻]影响——重建了低分辨率BWT记录，以验证上述推测。研究发现，北大西洋地区在1500 kyr至约500 kyr期间发生了约2~3℃的长期BWT降温，这与此前基于C. wuellerstorfi和O. umbonatus获得的BWT记录一致。同时我们还发现，MPT时期的大洋环流（ocean circulation）变化可能对d18O_benthic、BWT及海水δ¹⁸O记录产生影响。始于约1.2 Ma的北大西洋BWT降温（推测由高纬度冷却驱动），可能是约900 ka时气候-冰盖行为出现阈值响应的必要前驱条件。彼时冰体体积的适度增加与温盐环流重组（thermohaline reorganization），可能使得气候系统对100 kyr轨道周期（orbital cycle）的敏感性显著增强。