Collaborative Research: Uncovering marine carbon chemistry dynamics during the deglaciation with boron isotopes and radiocarbon

全球碳循环在大约12000年前的最后一个冰河期结束时经历了一次重大转变——当时大气中的二氧化碳含量突然上升。该项目将使用自然产生的“放射性碳”（放射性碳同位素14C）来研究全球碳库在冰川消融过渡期间的变化。该项目将测试当时东太平洋海底火山释放碳脉冲的放射性碳证据。这一结果挑战了固体地球碳释放相对缓慢和稳定的观点。对深海沉积物岩心化石壳的化学和同位素测量将显示碳脉冲是以酸还是碱的形式出现的。这将有助于确定碳的来源。碳循环模型将用于估计这些碳脉冲的规模及其对海洋酸度和大气二氧化碳水平的影响。初步数据和初步模型结果表明，碳源为酸/碱中性。如果是这样的话，那么大量的碳可以在不引起强烈的海洋酸化或大气二氧化碳急剧上升的情况下释放出来。二氧化碳是一种温室气体，人类活动导致了今天二氧化碳的大量释放。记录上一个冰河期结束时自然碳释放的规模和性质，将有助于预测人类碳释放对环境的影响。它还将有助于预测人类释放的碳是否会被自然过程中和。选择加利福尼亚湾的研究地点是因为它包含：(1)高质量的木材碎片年代学，(2)丰富且保存完好的底栖有孔虫，用于硼和碳同位素分析，(3)区域和时间上一致的14C异常的复制证据，以及(4)与东太平洋隆起相关的已知当地地质碳源。本次工作的主要内容来自d11B和B/Ca测量，以确定是否存在与14C异常相关的海水碳酸盐化学和/或pH变化，但建议的工作还包括更广泛的补充同位素地球化学测量（14C/C, d13C），以提高数据库的价值，作为对区域14C异常可能解释的约束。该项目还包括区域和全球碳循环模拟，以吸收多代理约束，并定量评估对全球海水碳化学和大气CO2的隐含影响。这项工作将为关于冰解碳释放对冰解二氧化碳上升的贡献的积极辩论提供信息。可行的假设是，地质碳释放的显著脉冲解释了观测到的14C异常，但不会对二氧化碳变化做出显著贡献，因为碳是以中和的碳酸氢盐离子形式出现的。总体而言，该研究将通过协作整合新颖的多代理测量与碳循环建模，在理解与人类相关的时间尺度上的自然碳循环变化方面发挥重要作用。两位pi都是早期的职业科学家，具有专业知识和业绩记录。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。

The global carbon cycle underwent a major transition at the end of the last ice age ~12,000 years ago – atmospheric carbon dioxide levels rose abruptly at that time. This project will use naturally-occurring “radiocarbon” (the radioactive carbon isotope 14C) to study how global carbon pools changed during that deglacial transition. The project will test previous radiocarbon evidence for a pulse of carbon release from seafloor volcanoes in the eastern Pacific Ocean at that time. That result challenges the idea that carbon release from the solid Earth is relatively slow and steady. Chemical and isotope measurements on fossil shells from deep-sea sediment cores will show whether the carbon pulse came in the form of acid or base. This will help pin down the source of the carbon. Carbon cycle models will be used to estimate the scale of these carbon pulses and their impact on ocean acidity and atmospheric carbon dioxide levels. Preliminary data and initial model results suggest that the carbon source was acid/base neutral. If so, then enormous quantities of carbon could have been released without causing strong ocean acidification or a strong atmospheric carbon dioxide rise. Carbon dioxide is a greenhouse gas and human activities are responsible for a large release of carbon dioxide today. Documenting the scale and nature of the natural carbon release at the end of the last ice age will help predict the environmental consequences of human carbon release. It will also help predict whether the human carbon release may be neutralized by natural processes.The Gulf of California study site was selected because it contains: (1) a high-quality wood fragment-based chronology, (2) abundant and well-preserved benthic foraminifera for boron and carbon isotope analysis, (3) replicated evidence for regionally and temporally coherent 14C anomalies, and (4) known local sources of geologic carbon associated with the East Pacific Rise. The main thrust of the proposed work comes from d11B and B/Ca measurements to establish if there was any seawater carbonate chemistry and/or pH change associated with the 14C anomalies, but the proposed work also includes a wider set of complementary isotope geochemical measurements (14C/C, d13C) to enhance the value of the database as a constraint on the possible explanations for the regional 14C anomalies. The project also includes regional and global carbon cycle modeling to assimilate the multi-proxy constraints and quantitatively assess the implied effects on global seawater carbon chemistry and atmospheric CO2. This work will inform the active debate about the contribution of deglacial carbon release to deglacial CO2 rise. The working hypothesis is that significant pulses of geologic carbon releases explain the observed 14C anomalies but would not significantly contribute to CO2 change because that carbon came in neutralized bicarbonate ion form. Overall, the study will take on a significant gap in understanding natural carbon cycle change on human-relevant timescales by collaboratively integrating novel multi-proxy measurements with carbon cycle modeling. Both PIs are early career scientists with demonstrated domain expertise and track record.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.