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海水中碳酸钙的沉淀和溶解。碳酸钙是构成石灰石和白垩岩等岩石的矿物。对这一组成部分的详细了解对于我们理解全球碳循环以及整个地球系统都很重要。这项提议旨在探索最近的发现，这些发现代表了我们对海洋无机碳预算的理解发生了根本性的、以前意想不到的变化。全球海洋中的大量碳酸钙是由海洋浮游生物产生的，特别是由球藻和有孔虫等微生物产生的。然而，这里的研究团队的一部分最近的研究突出了海洋硬骨鱼对海洋碳酸盐产量的重大额外贡献。它们都摄取海水，现在已知还会在它们的肠道内产生碳酸钙，并以非常高的速度排泄这些沉淀物，这是它们在海水中正常运作的一部分。这一以前未被认识到的海洋碳酸盐来源本身就很重要，但与对全球鱼类生物量的新估计相结合，很明显，它对海洋特定区域的碳酸盐产量做出了重大贡献，对全球碳酸盐产量的贡献较小，但意义重大。排泄后，这些碳酸盐沉淀物沉入公海(与其他远洋碳酸盐一起)，一旦它们到达碳酸盐的化学溶跃层(海水相对于碳酸钙变得不饱和的深度)，大部分物质将溶解。因此，这些晶体在开阔的海洋中可能的命运是在深处溶解。相比之下，热带浅海水域鱼类来源的沉淀物的命运虽然未知，但可能截然不同。由于鱼类产生的碳酸钙与海水温度呈正相关，热带地区较高的海洋温度将导致鱼类产生最高的沉淀物。此外，热带台地和陆架环境的浅层温暖水域的碳酸钙通常是饱和的，因此可以进行大量的碳酸盐保存和沉积物堆积。因此，在这些高生产率和低结晶溶解潜力的环境下，我们假设鱼源碳酸盐对浅水热带碳酸盐沉积物收支，特别是对热带碳酸盐沉积环境中往往具有重要体积重要性的细粒(碳酸盐泥浆)组分，做出了重要但以前未被认识的贡献。值得注意的是，在试图量化细粒碳酸盐沉积组分来源的情况下，相当大比例的泥质组分仍具有未知的来源(例如，巴哈马沉积物中的10%至40%)，但在形态上与以前认为的任何生物来源或无机来源不同。这个项目将检验这样一个假设，即鱼的碳酸盐在这一未知的碳酸盐泥浆中占了相当大的比例。我们将对四种具有代表性的浅水热带鱼种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在这些环境中积累碳酸盐沉积物。如果这种物质确实在这些环境中堆积，这将代表着热带浅水碳酸盐沉积物的一个以前未被识别的(并且可能在体积上重要的)来源。

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.