Department of Geology & Geophysics, Yale University, New Haven, CT 06511;
Department of Geology & Geophysics, Yale University, New Haven, CT 06511.
Proc Natl Acad Sci U S A. 2018 Feb 6;115(6):1174-1179. doi: 10.1073/pnas.1714744115. Epub 2018 Jan 22.
Past greenhouse periods with elevated atmospheric CO were characterized by globally warmer sea-surface temperatures (SST). However, the extent to which the high latitudes warmed to a greater degree than the tropics (polar amplification) remains poorly constrained, in particular because there are only a few temperature reconstructions from the tropics. Consequently, the relationship between increased CO, the degree of tropical warming, and the resulting latitudinal SST gradient is not well known. Here, we present coupled clumped isotope (Δ)-Mg/Ca measurements of foraminifera from a set of globally distributed sites in the tropics and midlatitudes. Δ is insensitive to seawater chemistry and therefore provides a robust constraint on tropical SST. Crucially, coupling these data with Mg/Ca measurements allows the precise reconstruction of Mg/Ca throughout the Eocene, enabling the reinterpretation of all planktonic foraminifera Mg/Ca data. The combined dataset constrains the range in Eocene tropical SST to 30-36 °C (from sites in all basins). We compare these accurate tropical SST to deep-ocean temperatures, serving as a minimum constraint on high-latitude SST. This results in a robust conservative reconstruction of the early Eocene latitudinal gradient, which was reduced by at least 32 ± 10% compared with present day, demonstrating greater polar amplification than captured by most climate models.
过去大气 CO 升高的温室时期的特点是全球海洋表面温度（SST）升高。然而，高纬度地区比热带地区升温幅度更大（极地放大）的程度仍然受到很大限制，特别是因为只有少数热带地区的温度重建。因此，CO 增加、热带地区变暖程度以及由此产生的纬度 SST 梯度之间的关系并不为人所知。在这里，我们提出了一组来自热带和中纬度全球分布地点的有孔虫的耦合聚类同位素（Δ）-Mg/Ca 测量结果。Δ 对海水化学不敏感，因此对热带 SST 提供了可靠的限制。至关重要的是，将这些数据与 Mg/Ca 测量结果相结合，可以精确重建整个始新世的 Mg/Ca，从而重新解释所有浮游有孔虫的 Mg/Ca 数据。综合数据集将始新世热带 SST 的范围限制在 30-36°C（来自所有盆地的站点）。我们将这些准确的热带 SST 与深海温度进行比较，作为对高纬度 SST 的最小限制。这导致了对早始新世纬度梯度的稳健保守重建，与现今相比，至少减少了 32±10%，表明极地放大程度比大多数气候模型所捕捉到的更大。
