Clumped isotope thermometry and oxygen isotope composition of seawater of key climate events during the Oligocene

了解过去的气候变化及其潜在原因需要重建过去的海洋成分和温度。然而，海洋温度和冰体积的传统代用指标（例如，有孔虫的稳定氧同位素和Mg/Ca值）受到生物过程或海洋化学变化的破坏。块状同位素测温法是一种很有前途的新方法，它绕过了这些问题。碳酸盐团块同位素测温完全基于热力学，因此与其他碳酸盐温度代用指标（氧同位素和Mg/Ca比值）相比，它提供的温度估计与源水的原始同位素或化学成分无关。因此，对碳酸盐样品的成对氧团同位素测量可以计算出碳酸盐沉淀的水的氧同位素组成。这意味着，团块同位素测温也能让我们深入了解全球冰体积的变化。这里使用的团块同位素方法允许测量小样本量，分析误差小至+/-1-2°C，与其他温度代理相当。渐新世是继古新世和始新世温暖的温室气候之后，冰库世界开始形成的重要气候期。渐新世的开始是全球气候最根本的重组之一，因为第一个永久性的大型大陆冰盖在南极洲形成。然而，最近在南大洋的一个地点使用团块同位素研究始新世-渐新世转变（EOT），表明在这个冰生长事件中没有明显的温度变化。这种意想不到的缺乏温度变化可能是由于低温下团块同位素- t校准的不确定性引起的。因此，本研究旨在建立南大洋高纬度地区现代地表、温跃层和底栖有孔虫的块状同位素-t定标，以完善已发表的低温端块状同位素定标方程。最后，本研究将在跨越不同纬度和海洋盆地的地点，通过成对测量有孔虫团块同位素值和Mg/Ca比值，重建可靠的海底和海面海水温度。这些分析将针对渐新世的特定时间间隔，如EOT（重新评估南大洋研究的结果）和关键的冰川期（Oi-2b, Mi-1）。i-2b冰期发生在整个渐新世冰量最大时期。在Mi-1冰期期间，底栖有孔虫氧同位素记录表明与EOT间隔相似的增加，提出了对冰增长的可比估计的问题。总之，研究这些区间将评估全球气候主要受南半球气候机制驱动时期的海水温度、海水氧同位素组成和全球冰量变化。

Understanding past climate changes and their underlying causes requires reconstructions of past ocean composition and temperatures. However, traditional proxies of ocean temperatures and ice volume (e.g., stabile oxygen isotopes and Mg/Ca values of foraminifera) are undermined by biological processes or changes in ocean chemistry. Clumped isotope thermometry, a promising, novel proxy, circumvents these problems. Carbonate clumped isotope thermometry is entirely thermodynamically based and therefore provides temperature estimates independent of the original isotopic or chemical composition of the source water, in contrast to other carbonate temperature proxies (oxygen isotopes and Mg/Ca ratios). Paired oxygen-clumped isotope measurements on carbonate samples thus enable the calculation of the oxygen isotope composition of the water, from which the carbonates precipitated. That means, clumped isotope thermometry also provides insights into global ice volume changes. The clumped isotope method used here allows measurement of small sample sizes and has an analytical error as small as +/-1-2°C, which is comparable to other temperature proxies. The Oligocene is an important climate epoch as it marks the inception of the icehouse world following the warm greenhouse climate of the Palaeocene and Eocene. The onset of the Oligocene is one of the most fundamental reorganisations of global climate, as the first permanent, large continental ice sheets formed on Antarctica. However, a recent study using clumped isotopes across the Eocene-Oligocene transition (EOT) at a Southern Ocean site indicates no evident temperature change across this ice growth event. This unexpected lack of temperature change could arise from uncertainties in the clumped isotope-T calibration at low temperatures. Therefore, this proposed study aims to establish a clumped isotope-T calibration of modern surface-, thermocline-dwelling and benthic foraminifera of high latitude Southern Ocean sites to improve published clumped isotope calibration equations on the low-temperature end. Finally, this study will reconstruct reliable bottom and surface seawater temperatures, using paired measurements of foraminiferal clumped isotope values and Mg/Ca ratios, at sites spanning different latitudes and ocean basins. These analyses will target specific time intervals of the Oligocene, such as the EOT (to reassess the findings of the Southern Ocean study) and key glaciation phases (Oi-2b, Mi-1). The Oi-2b glaciation occurred during maximum ice volume of the entire Oligocene. During the Mi-1 glaciation, benthic foraminiferal oxygen isotope records illustrate an increase similar to the EOT interval, raising the question of comparable estimates of ice growth. In summary, investigating these intervals will evaluate the seawater temperatures, seawater oxygen isotope composition and global ice volume changes during a time when global climate was mainly driven by Southern Hemisphere climate mechanisms.