Parameterizations for Organic Matter Oxidation Kinetics, 2007 (U.S. JGOFS Synthesis & Modeling Phase project results)

<p>Research topic: the cycling of organic carbon and calcium carbonate in marine sediments: Determination of parameters for use in global models</p> <p>Models attempting to explain past climate change and to predict changes in atmospheric composition and climate on time scales of 100 years or longer need to consider carbon cycling at the surface of marine sediments.&nbsp; Given the large CO₂ neutralization capacity of marine sediments, which is due to their high CaCO₃ content, it is particularly important to consider reactions affecting the preservation of calcite.&nbsp; The most important of these reactions are the oxic decomposition of organic matter and the calcite dissolution driven by benthic metabolism and by bottom water undersaturation.&nbsp; Through the efforts of JGOFS and other programs, a large data set describing reactions in the upper millimeters to centimeters of the sediment column has been created.&nbsp; This data set has not yet been examined to produce parameters that accurately describe organic matter decomposition and calcite dissolution.&nbsp; The work we propose is designed to use this new data set to generate parameters that can be used in models allowing extrapolation from studied areas to the global ocean.&nbsp; The ultimate result will be a substantially firmer foundation for global carbon mass balances.</p> <p>Our proposed approach to quantifying the role of benthic fluxes of organic carbon and CaCO₃ in the marine carbon cycle is to develop diagenetic models that describe organic matter degradation and CaCO₃ dissolution in sediments at the limited number of sites/regions where adequate data sets are available and to extrapolate the results from these sites to regional and global scales with the models generated.&nbsp; We have selected nine regions that best meet our data criteria and represent a wide range of environmental conditions in the oceans.</p> <p>Our parameterization of remineralization reactions will proceed in several steps.&nbsp; First, we will characterize the depth scales of organic matter oxidation within the sediments.&nbsp; The parameters resulting from this effort can be used directly to assess the role of metabolic acids in the dissolution of CaCO₃ at the sea floor.&nbsp; Second, at the more limited set of sites with high resolution pore water data as well as solid phase organic carbon and radiotracer data for bioturbation rate estimates, we can determine the time scales of organic matter decomposition.&nbsp; The latter results will then be used, in conjunction with the depth scale parameterization, to expand the geographic scope of our time scale determinations.&nbsp; Finally, we propose to examine the current parameterization of the rate law for calcite dissolution in sediments.&nbsp; In particular, we propose to carry out new experiments to determine the power of the dependence of dissolution rate on degree of saturation with respect to calcite.&nbsp; The end result of this data synthesis and parameterization effort will be greatly improved constraints on global estimates of the rate of carbon cycling at the sea floor.</p>研究主题：海洋沉积物中有机碳与碳酸钙的循环——全球模型用参数测定 旨在阐释过去气候变化、并预测百年及更长时间尺度上大气成分与气候变迁的模型，需考量海洋沉积物表层的碳循环过程。鉴于海洋沉积物因高碳酸钙（CaCO₃）含量而具备极强的二氧化碳中和能力，因此研究影响方解石保存的化学反应尤为关键。其中最为重要的两类反应为：有机质的有氧分解，以及由底栖代谢作用与底层水不饱和环境驱动的方解石溶解。依托全球海洋通量联合研究（JGOFS）及其他计划的工作成果，研究人员已构建起一套覆盖沉积物柱顶部数毫米至数厘米范围内反应过程的大型数据集。但目前尚未有研究基于该数据集生成可精准描述有机质分解与方解石溶解过程的参数。本研究拟利用这套全新数据集，生成可用于模型的参数，从而实现从研究区域向全球大洋的外推。最终研究成果将为全球碳质量平衡研究提供更为坚实的理论基础。 本研究拟通过以下方式量化有机碳与碳酸钙底栖通量在海洋碳循环中的作用：首先在具备充足数据集的有限站点/区域，构建可描述沉积物中有机质降解与碳酸钙溶解过程的成岩模型，再利用所生成的模型将研究结果外推至区域及全球尺度。我们已筛选出9个最符合数据标准的区域，这些区域涵盖了大洋中广泛的环境条件类型。 本次研究对再矿化反应的参数化工作将分多个步骤推进。第一步，我们将刻画沉积物内有机质氧化的深度尺度特征。本步骤所得参数可直接用于评估代谢酸在海底碳酸钙溶解过程中的作用。第二步，针对少量同时具备高分辨率孔隙水数据、固相有机碳数据以及可用于生物扰动速率估算的放射性示踪剂数据的站点，我们将确定有机质分解的时间尺度。后续将结合深度尺度参数化结果，拓展时间尺度测定的地理覆盖范围。最后，我们拟对当前沉积物中方解石溶解速率定律的参数化方案进行校验。具体而言，我们将开展全新实验，以明确溶解速率对方解石饱和程度的依赖指数。本数据整合与参数化工作的最终成果，将大幅提升全球海底碳循环速率估算结果的约束精度。