Chromium isotope (δ⁵³Cr) values of marine sedimentary rocks are increasingly utilised for monitoring changes in redox conditions in the Earth's ancient oceans and atmosphere. Carbonate rocks are of particular interest as they are relatively common in the sedimentary record and have been shown under laboratory conditions to be capable of recording the Cr isotope composition of precipitating fluids. However, little data from appropriate modern analogues is currently available to constrain the variability in δ⁵³Cr values within a carbonate platform and slope system and assess the preservation potential of marine δ⁵³Cr values during burial, lithification and diagenesis. In this paper we investigate the heterogeneity of δ⁵³Cr values on a modern marine carbonate platform in the context of carbonate sediment deposition, mineralogy, and the influence of burial and early lithification. We present new δ⁵³Cr values from carbonate sediments in the upper 200 m of ODP Leg 166 Site 1003 taken from the western margin of the Great Bahama Bank (GBB) and compare these data to modern carbonate sediments accumulating on the GBB platform-top. Shallow-water surface sediments record a range of heavy δ⁵³Cr values from +0.77 to +1.07‰ while drill core samples record a larger range of δ⁵³Cr values from +0.51 to +0.97‰. Down-core variability in δ⁵³Cr values occurs systematically with changes in sediment composition and mineralogy, where more positive δ⁵³Cr values occur in drill core samples that preserve proportionally more banktop-derived carbonate and less well developed diagenetic textures. These values decrease down-core where proportionally more pelagic carbonate exists, and with increasing neomorphism of primary aragonite to low-Mg calcite. The mineralogical dependence of δ⁵³Cr values at Site 1003 appears to be related to variations in relative sea-level across the platform and the degree of lithification and diagenetic recrystallization with depth, rather than in response to changes in the δ⁵³Cr composition of the overlying water column. As the GBB represents a reasonable modern analogue for many Phanerozoic and Precambrian carbonate successions, the preliminary framework presented here may have significant implications for the interpretation of vertical profiles of δ⁵³Cr values in paleo-redox studies. In particular, smaller scale variability in ancient carbonate sections can be more simply explained by local variations in depositional environment and diagenetic regime rather than fluctuations in water column redox conditions.

铬同位素（δ ⁵³ Cr）的海洋沉积岩的价值越来越多地用于监测地球的古海洋和大气氧化还原条件的变化。碳酸盐岩特别受关注，因为它们在沉积记录中相对常见，并且在实验室条件下已被证明能够记录沉淀流体的Cr同位素组成。然而，从适当的现代类似物小数据是目前可用的，以限制在δ的可变性^53个碳酸盐平台和斜坡系统内Cr值，并评估海洋δ的保存潜力^53个中掩埋，岩化和成岩Cr值。在本文中，我们探讨δ的异质性⁵³在碳酸盐沉积物沉积，矿物学以及埋藏和早期石化作用的影响下，现代海洋碳酸盐平台上的Cr值。我们提出了新的δ ⁵³铬碳酸盐沉积物航次166网站1003从大巴哈马银行（GBB）西缘采取的鞋面200 M值和比较这些数据，以现代碳酸盐沉积物堆积在GBB平台之上。浅水表面沉积物记录范围重δ的⁵³从0.77至1.07 +‰而岩芯样本记录较大范围δ的Cr值^53个Cr值从0.51到0.97 +‰。在δ向下核变性^53个Cr值与沉积物组成和矿物学，其中更积极的δ的变化系统地发生⁵³Cr值出现在钻芯样品中，这些样品按比例保留了更多的源自岸上碳酸盐的碳酸盐和较少发育的成岩质地。这些值降低了下层碳酸盐岩成比例存在的下岩心，并随着初级文石向低镁方解石的新晶型的增加而增加。δ的矿物学依赖性⁵³ Cr值在站点1003出现在δ涉及到在整个平台和岩化和重结晶成岩随深度的程度相对海平面的变化，而不是响应于变化⁵³上方水柱的Cr组成。由于GBB代表了许多显生宙和前寒武纪碳酸盐继承合理的现代模拟，初步框架提出这里可能有针对δ的垂直剖面解释显著影响⁵³在古氧化还原研究Cr值。特别是，古代碳酸盐岩剖面中较小的尺度变化可以通过沉积环境和成岩作用的局部变化而不是水柱氧化还原条件的波动来更简单地解释。