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

要了解过去的气候变化及其根本原因，需要重建过去的海洋成分和温度。然而，传统的海洋温度和冰量指标(如稳定的氧同位素和有孔虫的镁/钙比值)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替代品，它绕过了这些问题。与其他碳酸盐温度替代指标(氧同位素和镁/钙比)不同，碳酸盐聚集同位素测温法完全基于热力学，因此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，在碳酸盐样品上进行配对的氧团同位素测量，可以计算出碳酸盐从水中析出的氧同位素组成。这意味着，成束同位素测温技术也提供了对全球冰量变化的洞察。这里使用的块状同位素方法允许测量小样品尺寸，分析误差小至+/-1-2°C，可与其他温度替代方法相媲美。渐新世是一个重要的气候时期，因为它标志着继古新世和始新世的温暖温室气候之后冰库世界的开始。渐新世的开始是全球气候最根本的重组之一，因为南极洲形成了第一个永久的大型大陆冰盖。然而，最近在南大洋的一个地点利用始新世-渐新世转换(EOT)期间的块状同位素进行的研究表明，在这一冰生长事件中没有明显的温度变化。这种意想不到的温度变化的缺乏可能是由于低温下聚集的同位素-T校准中的不确定性造成的。因此，这项拟议的研究旨在建立现代高纬度南大洋表层、温跃层和海底有孔虫的块状同位素-T定标，以改进已发表的低温端的块状同位素定标方程。最后，这项研究将在跨越不同纬度和海洋盆地的地点，通过配对测量有孔虫聚集的同位素值和镁/钙比，重建可靠的海底和表层海水温度。这些分析将针对渐新世的特定时间间隔，例如EOT(重新评估南大洋研究的结果)和关键冰川阶段(Oi-2b，Mi-1)。OI-2b冰期发生在整个渐新世冰量最大的时期。在米-1冰期，海底有孔虫的氧同位素记录显示出类似于EOT间期的增加，这引发了对冰增长的可比估计的问题。总之，调查这些间隔将评估在全球气候主要由南半球气候机制驱动的时期内的海水温度、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.