The role of ocean crust low temperature alteration carbonates in the global calcium cycle

洋壳玄武岩低温蚀变过程中形成的碳酸钙沉淀（LTA碳酸盐）是全球钙循环中重要的钙汇。据估计，总钙通量约占海洋总钙输出通量的10%。对LTA碳酸盐的钙同位素组成的首次测量表明存在明显的分馏。这表明这种钙通量对全球海洋钙同位素收支有可测量的影响。LTA碳酸盐代表了我们对全球钙同位素通量知识的空白，这可能解释了新近纪同位素收支的差异。因此，我们计划对不同地点、不同地壳深度和不同地壳年龄的LTA碳酸盐岩进行取样。我们将使用氧同位素、微量元素和锶同位素来限制这些沉淀物的钙来源。然后，我们将测量LTA碳酸盐的钙同位素组成，以估计从海洋到这个重要汇的全球平均同位素通量。因此，该项目将有助于更好地限制钙同位素通量，并找到当前钙同位素预算差异的解释。这些数据将进一步对不同地壳深度LTA碳酸盐岩的形成条件提供新的认识。他们将最终证明这些形成条件是否随时间而变化，以及LTA碳酸盐是否可以作为海水钙同位素组成的记录器。一个补充项目（a . kl<e:1> gel， W. Bach; Univ. Bremen）有望为解释我们的数据提供有价值的补充信息，该项目旨在将IODP/ODP岩心中LTA碳酸盐的分布清单与这些碳酸盐脉的微量元素组成和年龄结构的精细调查相结合。因此，我们计划联合抽样并调查共同样本，以保证两个项目的协同作用和数据的可比性。

Calcium carbonate precipitate that form during the low temperature alteration of ocean crust basalts (LTA carbonates) are an important calcium sink in the global calcium cycle. The total calcium flux has been estimated as representing about 10 % of the total Ca output flux of the ocean. First measurements of the calcium isotopic composition of LTA carbonates point to a significant fractionation. This indicates a measureable influence of this calcium flux on the global calcium isotope budget of the oceans. LTA carbonates represent a gap in our knowledge of the global calcium isotope fluxes that may explain discrepancies in the Neogene isotope budget. We therefore plan to sample LTA carbonates from different sites, different crustal depths and different crustal ages. We wil) use oxygen isotopes, trace elements and strontium isotopes to constrain the sources of calcium for these precipitates. We will then measure the calcium isotopic composition of the LTA carbonates to estimate the globally averaged isotope flux from the ocean to this important sink. With that the project will help to better constrain calcium isotope fluxes and to find an explanation for the discrepancies in current calcium isotope budgets. The data will further give new insights in the formation conditions of LTA carbonates at different crustal depths. They will finally show whether these formation conditions varied with time and whether LTA carbonates may be used as recorders of the ocean water calcium isotope composition through time. Valuable supplemental information for the interpretation of our data is expected from a complementary project (A. Klügel, W. Bach; Univ. Bremen) which aims to combine an inventory of the distribution of LTA carbonates in IODP/ODP cores with a fine-scale investigation of the trace elemental composition and age structure of these carbonate veins. Therefore we plan joint sampling sessions and to investigate common samples to guarantee the synergy of the two projects and the comparability of the data.