Collaborative Research: Provenance of Alkenones & Holocene Temperature Evolution of the NW Atlantic

该项目将沿北大西洋西部生成一套12个新的海面温度重建图，以检验基于烯烃的海洋温度记录的准确性。过去几千年的温度数据显示，基于陆地、海洋和气候模型的估算存在不匹配。气候模式显示温度保持相对不变，而陆地记录显示气温略有上升。奇怪的是，海洋表面温度在同一时期下降了4-6°C。但人们担心这些记录在多大程度上反映了该地区的温度状况。烯酮是由藻类产生的，可能会受到藻类最丰富的季节温度的影响。它们也可以被洋流从其他地方带到北大西洋，并反映出这些地区的温度状况。这项研究将试图通过检查全年在水柱中直接收集的烯酮，并将这些烯酮与沉积在附近海底的烯酮进行比较，来解决这些问题。氢同位素比率和放射性碳数据将提供一种测试，以了解有多少物质从不同的来源输送过来。烯烃温度数据将与该地区其他温度指标（如有孔虫的Mg/Ca）进行比较。该项目为初中和高中教师创造了重要的学习机会，他们将在课堂上与科学家合作开发基于气候的课程教学，并在沉积物取心作业期间参与船上研究。它还将为本科生和早期职业博士后提供重要支持。全球全新世温度替代记录的综合表明，在过去8000年里，北半球海洋记录的温度下降了2°C，这在很大程度上推动了0.5-1°C的降温。然而，短暂的气候模拟并没有显示出这种冷却，北美东部和欧洲的大陆温度记录显示出轻微的变暖。这个“全新世难题”是气候科学界面临的最突出的未解决的挑战之一。在西北大西洋的三个烯烃海温记录中，北半球的趋势主要是由4-6°C的降温所驱动的。为了确定这种数据模式的不匹配是否表明对气候系统的理解存在差距，必须在影响西北大西洋斜坡水域烯烃沉积的因素的背景下检查这些异常代理记录，并将其与同一地点的其他海温代理进行比较，这些海温代理目前缺乏样品和分析。本研究将通过测量2004-2007年在西北大西洋坡水区收集的沉积物岩心和存档沉积物捕集器样本中烯烃的氢同位素值和放射性碳年龄，以及浮游有孔虫中的Mg/Ca比率来解决这两个缺陷。烯烃海温记录可能受到缓慢下沉的烯烃通过表层和深海洋流的横向输送或季节性生产的影响，这两者都可能导致数据模型不匹配。该项目将利用西北大西洋地表水中较大的氢同位素梯度，通过测量烯酮中的2H/1H比率来确定烯酮的合成来源。来源将通过测量烯酮14C年龄进一步评估，以确定平流期间预老化的程度。将烯酮温度和14C年龄与快速下沉、有孔虫Mg/Ca温度和方解石14C年龄在一系列地点和深度上的季节性分辨率进行比较，将使我们能够全面表征相对于原位和卫星海温的平流或季节性偏置。在确定了这些潜在偏差的影响之后，将通过在新泽西州和新斯科舍省以东的6个计划取样点的12个新重建（每个6个来自两个独立代理）对西北大西洋全新世海温进行强有力的重新评估，从而为“全新世难题”提供可能的解决方案。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This project will produce a set of 12 new sea-surface temperature reconstructions along the western North Atlantic to examine the accuracy of alkenone-based ocean temperature records. Temperature data over the past several thousand years show a mismatch between land-, sea-, and climate model-based estimates. Climate models suggest that temperature remained relatively unchanged, while land-based records shows slight warming. Oddly, sea surface temperatures show 4-6°C cooling over the same period of time. But concerns exist about how well these records reflect temperature conditions in the region. Alkenones are produced by algae which may be biased by the temperature of the season when algae are most abundant. They can also be carried to the North Atlantic from elsewhere by ocean currents and reflect temperature conditions of those regions. This study will attempt to resolve these issues by examination of alkenones collected directly in the water column throughout the year and comparing these to alkenones deposited on the seafloor nearby. Hydrogen isotope ratios and radiocarbon data will provide a test of how much material is transported from different sources. Alkenone temperature data will be compared to other temperature proxies (e.g. Mg/Ca of foraminifera) in the region. The project creates significant learning opportunities for middle and high school teachers who will team with scientists in the classroom to develop climate-based curricular instruction and participate in shipboard research during sediment coring operations. It will also provide significant support for undergraduate students and an early career postdoctoral student. The global composite of Holocene temperature proxy records indicates 0.5-1°C of cooling over the last 8,000 years, driven largely by a 2°C cooling in northern hemisphere marine records. Transient climate simulations, however, show no such cooling, and continental temperature records in eastern North America and Europe indicate slight warming. This “Holocene Conundrum” is one of the most prominent unmet challenges facing the climate science community. The northern hemisphere trend is largely driven by 4-6°C cooling in three alkenone SST records from the NW Atlantic. To determine whether this data-model mismatch indicates a gap in understanding of the climate system, these anomalous proxy records must be examined in the context of the factors influencing alkenone deposition in NW Atlantic slopewaters and compared to other SST proxies in the same locations for which samples are not available and analyses are presently lacking. Both deficiencies will be addressed in this research by measuring hydrogen isotope values and radiocarbon ages of alkenones, along with Mg/Ca ratios in planktonic foraminifera, from sediment cores and archived sediment trap samples collected in 2004-2007 in the NW Atlantic slopewater region. Alkenone SST records can be biased by lateral transport of slow-sinking alkenones via surface and abyssal currents, or by seasonal production, both potential causes for the data-model mismatch. This project will take advantage of the large hydrogen isotope gradient in NW Atlantic surface waters to ascertain alkenone synthetic provenance by measuring 2H/1H ratios in alkenones. Provenance will be further assessed by measuring alkenone 14C ages to determine the extent of pre-aging during advection. Comparison of alkenone temperatures and 14C ages to fast-sinking, foraminiferal Mg/Ca temperatures and calcite 14C ages, across a series of locations and depths, with seasonal resolution, will allow for the full characterization of advective or seasonal biasing in either proxy relative to in situ and satellite SSTs. Having established the impact of these potential biases, a robust re-assessment of NW Atlantic Holocene SSTs will be made via 12 new reconstructions (6 each from the two independent proxies) from 6 planned coring sites on depth transects east of New Jersey and Nova Scotia, allowing a possible solution to the “Holocene conundrum”.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.