Improving the suitability of the polar to subpolar planktic foraminifera N. pachyderma as a climate archive: New approaches to deduce 'near surface' temperatures

第 1 部分：极地地区对气温上升和大气温室气体浓度增加特别敏感。目前，观测到的北极变暖几乎是整个北半球变暖的两倍。一个主要担忧是，该地区的进一步变暖可能会削弱北大西洋环流，这将对欧洲和北美的气候产生深远的影响。为了更好地了解控制北半球未来气候变化速度和幅度的机制，我们需要从该地区过去的气候事件中获取可靠的数据，这些数据可以与当前的观测结果进行比较，并有助于未来的预测。我们对地球过去气候的大部分了解都是通过对从钻芯中回收的化石有孔虫壳的化学分析得出的。有孔虫在世界海洋中无处不在，是一种形成沙粒大小的碳酸钙（方解石）壳的小型生物。在生命周期结束时，空壳会沉积在海底，可以保存数千万年。虽然从 20 世纪 50 年代开发出来的有孔虫贝壳化石重建过去的气候事件的方法，并在几十年来不断改进，在低纬度和中纬度海洋中效果很好，但从北大西洋极地的有孔虫贝壳中推断出准确的过去温度仍然具有挑战性。本研究从北大西洋极地获得的有孔虫贝壳化石（Neogloboquadrina pachyderma 种）的初步数据表明，所谓的“层状方解石”是在上层水柱中形成并提供“近地表”信号的早期贝壳方解石，通常部分或大部分溶解。因此，剩下的壳主要由后来形成的外部“地壳方解石”组成，它记录了更大（通常更冷）水深的环境条件。由于受影响的贝壳看起来没有可疑之处（通过显微镜检查时从外部看不到层状方解石的损失），因此在许多先前的研究中可能会选择失去层状方解石的贝壳进行分析。我们假设由溶解的层状方解石的贝壳重建的温度偏向于较冷的温度。幸运的是，在同一岩心样本中，保存完好的层状方解石贝壳通常与保存不良的贝壳同时存在，因此可以选择进行分析。本研究旨在仔细选择具有完整层状方解石（保留近地表信号）的贝壳，并测量层状方解石的镁/钙比率作为过去温度的化学记录器。这与传统的分析方法存在显着偏差，传统的分析方法使用未知比例的层状方解石（温暖的近地表信号）和地壳方解石（冷的深水信号）来平均整个贝壳的成分。将在本研究框架内开发的分析方法有可能显着提高北大西洋极地古记录的准确性，这对于衡量这一敏感地区对未来温室气体强迫的响应至关重要。第 2 部分：北大西洋极地的气候变化速度远高于世界海洋的大多数其他区域。因此，准确记录该地区过去的气候变化对于改善对这一敏感地区在未来温室气体强迫增强的情况下如何反应的预测至关重要。然而，即使经过几十年的研究，北大西洋极地的古气候记录仍然没有解答许多问题，而且往往与模型模拟的结果不一致。反映过去气候变化的绝大多数海面温度记录来自浮游有孔虫的化石壳；世界海洋中普遍存在的微小单细胞生物。有孔虫的一个特征是其沙粒大小的碳酸钙（方解石）壳，最终沉积在海底，可以保存数千万年。化石贝壳可以从钻芯中回收，随后对其化学或同位素成分进行分析，可以重建贝壳形成时持续的水温（和其他参数）。这种重建古气温的方法在极地以外的地区也非常有效。本研究的主要目标之一是评估有孔虫物种 Neogloboquadrina pachyderma（高纬度沉积物中唯一丰富的有孔虫）的某些特征是否有助于从北大西洋极地推断出可靠的古记录。这项研究获得的先导数据表明，厚皮猪笼草贝壳化石中所谓的内部“层状方解石”，即在上层水柱（通常温度较高）中形成的早期生成的方解石，通常大部分被溶解，而外部“地壳方解石”，在更深的水域中形成，因此通常反映较冷的温度，仍然保存完好。有趣的是，当用显微镜检查贝壳时，从外部看不到层状方解石的这种损失。因此，在许多先前的研究中，很可能选择含有溶解的层状方解石的贝壳进行分析，这可能使计算的温度偏向于形成剩余外壳的较冷、较深的水域。幸运的是，我们发现保存完好的层状方解石贝壳通常与保存不良的贝壳同时存在。本研究的目的是开发新的筛选方法来仔细筛选保存完好的层状方解石的厚皮猪笼草贝壳。随后，将通过激光烧蚀分析在海面附近形成的层状方解石的镁/钙（Mg/Ca）比率，该比率可以转换为过去有孔虫形成壳时持续的水温。这与使用整个壳的传统方法存在显着偏差，在层状方解石（近地表信号）中记录的信息与地壳方解石（深水信号）的成分混合。因此，传统的分析方法可能无法捕获海面或海面附近的整个温度变化。将在本研究框架内开发的分析方法有可能显着提高北大西洋极地古记录的准确性，这对于衡量这一敏感地区对未来温室气体强迫的响应至关重要。这项新技术将在北大西洋极地的两个沉积物岩心上进行进一步测试，跨越过去约 26,000 年的时间，从最后一个冰河时代的最冷温度到现代温暖得多的气候。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Part 1: Polar regions are particularly sensitive to rising temperatures and increasing concentrations of atmospheric greenhouse gases. Currently, the observed warming in the Arctic is almost twice that of the Northern Hemisphere as a whole. A major concern is that further warming of this region could potentially weaken the North Atlantic circulation, which will have a profound influence on climate in Europe and North America. In order to gain a better understanding of the mechanisms that control the rate and magnitude of future climate change on the Northern Hemisphere, we need to acquire robust data from past climate events from this region that can be compared to current observations and help with future predictions. Much of our knowledge of the Earth’s past climate is deduced from chemical analysis of fossil foraminifera shells recovered from drill cores. Foraminifera, which are ubiquitous in the World’s Oceans, are small organisms that form a sand-grain-sized calcium carbonate (calcite) shell. At the end of their life cycle, the empty shell is deposited on the seafloor where it can be preserved for tens of millions of years. While the approach to reconstruct past climate events from fossil foraminifera shells, which was developed in the 1950s and continuously improved over many decades, works well in low- and mid-latitude oceans, deducing accurate past temperatures from foraminifera shells from the Polar North Atlantic remains challenging. Pilot data from fossil foraminifera shells (species Neogloboquadrina pachyderma) from the Polar North Atlantic acquired for this study reveal that the so-called ‘lamellar calcite’, which is the early generation of shell calcite that forms in the upper water column and provides a ‘near-surface’ signal, is often partially or largely dissolved. Thus, the remaining shell comprises mostly the later-formed outer ‘crust calcite’, which records environmental conditions from much greater (typically colder) water depths. As affected shells look unsuspicious – the loss of the lamellar calcite is not visible from the outside when inspected by microscope – shells that have lost their lamellar calcite have likely been selected for analysis in many prior studies. We hypothesize that temperature reconstructions from shells with dissolved lamellar calcite are biased towards colder temperatures. Fortunately, shells with well-preserved lamellar calcite are typically co-occurring with those featuring poor preservation in the same core samples and can thus be selected for analysis. This study aims to carefully select shells with intact lamellar calcite (preserved near-surface signal), and the ratio of Magnesium/Calcium of the lamellar calcite as a chemical recorder of past temperatures will be measured. This is a significant deviation from conventional analytical approaches that average the composition of the whole shell with an unknown proportion of lamellar calcite (warm near-surface signal) and crust calcite (cold deep-water signal). The analytical approach, which will be developed within the framework of this study, has the potential to significantly improve the veracity of paleorecords from the Polar North Atlantic, which is essential to gauge the response of this sensitive region to future greenhouse gas forcing. Part 2: Climate change in the polar North Atlantic occurs at a much higher rate than most other areas of the World’s Oceans. Therefore, accurate records of past climate variability from this area are essential to improve forecasts of how this sensitive region will react in future scenarios with enhanced greenhouse gas forcing. However, even after decades of research, paleoclimate records from the polar North Atlantic leave many questions unanswered and often do not align with results from model simulations. The vast majority of sea surface temperature records, reflecting past climate variability, are derived from fossil shells of planktic foraminifera; minute, single-celled organisms which are ubiquitous in the World’s Oceans. A characteristic feature of foraminifera is their sand-grain-sized calcium carbonate (calcite) shell which is eventually deposited on the seafloor where it can be preserved for tens of millions of years. Fossil shells can be recovered from drill cores, and subsequent analyses of their chemical or isotopic composition allows for the reconstruction of the water temperature (and other parameters) that persisted while the shells were formed. This approach to reconstruct paleotemperatures works very well outside the polar regions. One of the main objectives of this study is to assess whether certain characteristics of the foraminifera species Neogloboquadrina pachyderma, which is the only abundant foraminifera in high latitude sediments, contribute to the challenges to deduce robust paleorecords from the Polar North Atlantic. Pilot data acquired for this study indicate that the so-called inner ‘lamellar calcite’ of fossil N. pachyderma shells, which is the early generation of calcite that forms in the upper water column (typically warmer temperatures), is often largely dissolved, whereas the outer ‘crust calcite’, which forms in much deeper waters and thus typically reflects colder temperatures, is still well preserved. Interestingly, this loss of the lamellar calcite is not visible from the outside when the shells are inspected by microscope. Thus, it is likely that shells with dissolved lamellar calcite were selected for analysis in many prior studies, which may have biased the calculated temperatures towards the cooler, deeper waters where the remaining outer crust was formed. Fortunately, we have found that shells with well-preserved lamellar calcite are typically co-occurring with those featuring poor preservation.The objective of this study is to develop new screening methods to carefully select N. pachyderma shells featuring well-preserved lamellar calcite. Subsequently, the lamellar calcite, which has formed near the sea surface, will be analyzed by laser-ablation for its Magnesium/Calcium (Mg/Ca) ratio, which can be converted to the water temperatures that persisted in the past when the foraminifera formed their shells. This is a significant deviation from conventional approaches that use the entire shell, whereby the information recorded in the lamellar calcite (near-surface signal) is mixed with the composition of the crust calcite (deep water signal). Thus, conventional analytical approaches may not capture the entire temperature variation at or near the sea surface. The analytical approach, which will be developed within the framework of this study, has the potential to significantly improve the veracity of paleorecords from the Polar North Atlantic, which is essential to gauge the response of this sensitive region to future greenhouse gas forcing. This new technique will further be tested on two sediment cores from the Polar North Atlantic, spanning the past ~26,000 years from the coldest temperatures of the last ice age to the modern, much warmer climate.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.