Calcium Carbonate Dissolution Mechanisms in the Ocean

碳酸钙在海水中的溶解速度是海洋碳循环的重要组成部分。这是海洋吸收化石燃料二氧化碳能力的关键因素。这对理解和预测海洋酸化也很重要，并可能在遏制气候变化的新技术中发挥作用。尽管如此，碳酸钙溶解的机理和速率仍有很大的不确定性。本项目将采用新方法进行实验室实验，研究几种不同条件下碳酸钙的溶解。该项目将支持两名早期职业研究人员，其中包括一名研究生。结果将与公众分享，包括通过动画艺术家的支持。在之前的工作中，这个团队和他们的合作者已经对我们对海洋中碳酸盐溶解速率规律的理解进行了一些改进。在实验室和水柱中使用一种新型的碳同位素示踪技术，他们可以比其他技术更灵敏地测量溶解速率20-200倍。在接近平衡状态时尤其如此，因为那里的速率较慢，而且大部分海洋都在那里。他们之前的工作表明，速率定律在三种不同的表面溶解机制模式之间有两个临界阈值：接近平衡时，溶解随着台阶边缘的推进而进行；随着欠饱和程度的加深，溶液中的自由能在固体晶格缺陷处打开快速溶解的蚀刻坑；在远离平衡态的地方，蚀刻坑开始均匀地在二维固体表面形成，导致最大的溶解速率。然而，对速率定律的完全化学控制及其与这些表面能特征的相互作用的理解尚未确定。该项目将寻求从实验和理论上限制海洋碳酸盐溶解速率定律背后的机制。饱和状态将作为化学的其他方面的功能被系统地改变，以测试多年来成功地用于描述淡水溶解的基本反应。在独立实验中，饱和状态会随着硫酸盐浓度、钙/碳酸盐比例和镁/钙比例的变化而变化。原始数据的初始时间依赖性和实验固体的SIMS微探针剖面将允许确定总沉淀和总溶解速率，并结合溶解碳同位素比值随时间产生的斜率的净速率约束。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 rate at which calcium carbonate dissolves in seawater is an important piece of the ocean carbon cycle. It is a key factor in the oceans’ ability to absorb fossil fuel carbon dioxide. It is also important to understanding and predicting ocean acidification and may play a role in new technologies to curb climate change. Despite this importance, a great deal remains uncertain about the mechanism and rate of calcium carbonate dissolution. This project will conduct laboratory experiments using new methods to study calcium carbonate dissolution at several different conditions. The project will support two early-career investigators including a graduate student. Results will be shared with the public, including through support of an animation artist. In previous work, this team and their collaborators have made several improvements to our understanding of the carbonate dissolution rate law in the ocean. Using a novel carbon isotope tracer technique in the lab, and in situ in the water column, they can measure the dissolution rate 20-200x more sensitively than other techniques. This is especially true near equilibrium where rates are slow, but also where most of the ocean resides. Their previous work demonstrated that the rate law has two critical thresholds between three separate modes of surface dissolution mechanisms: near equilibrium, dissolution proceeds as the advance of step edges; with further undersaturation, the free energy in solution opens fast-dissolving etch pits at lattice defects in the solid; furthest from equilibrium, etch pits begin to form homogeneously on the 2-D solid surface leading to maximal dissolution rates. However, understanding of the full chemical control on the rate law, and its interplay with these surface energetic features has yet to be determined. This project will seek to experimentally and theoretically constrain the mechanism behind the rate law for marine carbonate dissolution. The saturation state will be systematically changed as a function of other aspects of the chemistry to test the elementary reactions that have been successfully used for years to describe dissolution in freshwaters. The saturation state will be varied over a range of sulfate concentrations, calcium/carbonate ratios, and magnesium/calcium ratios in independent experiments. Both the initial time dependence of the raw data, and SIMS microprobe profiles of the experimental solids will allow determination of the gross precipitation and gross dissolution rates in conjunction with the net rate constraints from slopes of dissolved carbon isotope ratios produced versus time.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.