The production and fate of fish-derived carbonate crystals in tropical shallow marine environments

海洋-大气碳循环的一个主要组成部分是碳酸钙在海水中的沉淀和溶解。碳酸钙是构成石灰岩和白垩等岩石的矿物。对这一成分的详细了解对我们理解全球碳循环和整个地球系统都很重要。本提案旨在探索最近的发现，这些发现代表了我们对海洋无机碳收支的理解的根本和以前意想不到的变化。在全球海洋中，大量的碳酸钙是由海洋浮游生物产生的，特别是由微生物，如颗石藻和有孔虫产生的。然而，这里的部分研究小组最近的研究强调了海洋硬骨鱼对海洋碳酸盐生产的重要额外贡献。它们都摄取海水，现在已知它们也在肠道内产生碳酸钙并以很高的速度排出这些沉淀物，这是它们在海水中的正常功能的一部分。这种以前未被认识到的海洋碳酸盐来源本身是重要的，但是，当与全球鱼类生物量的新估计相结合时，很明显，它对海洋特定区域的碳酸盐生产做出了重大贡献，而对全球的贡献较小，但意义重大。在排泄之后，这些碳酸盐沉淀物（与其他远洋碳酸盐一起）沉入开阔的海洋，一旦它们到达碳酸盐的化学溶解石（海水相对于碳酸钙不饱和的深度），这些物质中的大部分就会溶解。因此，这些晶体在开阔的海洋中可能的命运是在深海中溶解。相比之下，热带浅海海水中鱼类沉淀的命运虽然未知，但可能大不相同。由于鱼类的碳酸钙产量与海水温度正相关，热带地区较高的海洋温度将导致鱼类产生最高的沉淀率。此外，热带台地和陆架环境的浅层温暖水域通常碳酸钙饱和，从而允许大量碳酸盐保存和沉积物积聚。因此，在这些具有高产量和低晶体溶解潜力的环境中，我们假设鱼类衍生的碳酸盐对浅水热带碳酸盐沉积预算，特别是对热带碳酸盐沉积环境中通常具有体积重要性的细粒（碳酸盐泥）组分做出了重要的贡献，但以前未被认识到。值得注意的是，在试图量化细粒碳酸盐沉积物组分来源的地方，有相当大比例的泥组分仍然是未知来源（例如，在巴哈马沉积物中占10%至40%），但在形态上与以前认为的任何生物成因或无机来源不同。该项目将验证一种假设，即鱼类碳酸盐构成了这一未知碳酸盐泥浆的很大一部分。我们将对四种具有代表性的浅水热带鱼产生的晶体的超微结构和元素特征进行评估，并确定这些晶体是否有助于这些环境中的碳酸盐沉积物积聚。如果这种物质确实在这些环境中积累，这将代表一种以前未被认识到的热带浅水碳酸盐沉积物的来源（并且可能具有重要的体积意义）。

A major component of the marine-atmospheric carbon cycle is the precipitation and dissolution of calcium carbonate in seawater. Calcium carbonate is the mineral that makes up rocks such as limestone and chalk. Detailed knowledge of this component is important to our understanding of the global carbon cycle, and to the earth system as a whole. This proposal aims to explore recent findings that represent a fundamental and previously unexpected change to our understanding of the marine inorganic carbon budget. Large amounts of calcium carbonate are produced in the global oceans by marine plankton and, specifically, by microscopic organisms such as coccolithophores and foraminifera. However, recent research by part of the research team here has highlighted the significant additional contribution to oceanic carbonate production by marine bony fish. These all ingest seawater and are now known to also produce calcium carbonate within their guts and excrete these precipitates at very high rates as part of their normal functioning in seawater. This previously unrecognised source of marine carbonate is significant in its own right but, when combined with new estimates of global fish biomass, it is clear that it makes a major contribution to carbonate production in particular regions of the ocean, and a smaller but significant contribution globally. Following excretion, these carbonate precipitates sink in the open oceans (along with other pelagic carbonates) and, once they reach the chemical lysoclines for carbonate (the depth at which ocean waters become undersaturated with respect to calcium carbonate), much of this material will dissolve. Thus the likely fate of these crystals in the open oceans is to dissolve at depth. In contrast, the fate of fish-derived precipitates in shallow tropical marine waters, whilst unknown, may be very different. Because fish production of calcium carbonate is positively correlated with seawater temperature, higher marine temperatures in the tropics will result in highest rates of precipitate production by fish. In addition, the shallow warm waters of tropical platform and shelf environments are typically saturated with respect to calcium carbonate, thus allowing substantial carbonate preservation and sediment accumulation to occur. Thus in these settings, with both high production rates and less potential for crystal dissolution, we hypothesise that fish-derived carbonates are making an important, but previously unrecognised, contribution to shallow water tropical carbonate sediment budgets and, especially, to the fine-grained (carbonate mud) fractions that are often volumetrically important in tropical carbonate depositional environments. It is significant to note that where attempts have been made to quantify fine-grained carbonate sediment fraction sources a significant proportion of the mud fraction remains of unknown origin (e.g. between 10 and 40% in Bahamian sediments) but is morphologically distinct from that attributable to any previously considered biogenic or inorganic source. This project will test the hypothesis that fish carbonates make up a substantial portion of this unknown fraction of carbonate mud. We will undertake an assessment of the ultrastructural and elemental characteristics of crystals that are produced by four representative shallow water tropical fish species and determine whether such crystals are contributing to carbonate sediment accumulation in these environments. If this material is indeed accumulating in these environments this would represent a previously unrecognised (and potentially volumetrically important) source for tropical shallow water carbonate sediments.