Stable carbon isotope record of Bahama Bank sediments@en

The carbon-isotopic composition (d13C) of bulk carbonates, obtained from a transect of sites drilled through platform and periplatform sediments of Holocene to Early Miocene age, has been compared to ascertain whether changes in the d13C can be correlated between sediments of equivalent ages and whether such changes can be related to global changes in the d13C of the dissolved inorganic carbon in the oceans over this time period. Five of the sites were drilled during Leg 166 of the Ocean Drilling Project (1003–1007) in a transect ranging from five km to 25 km away from the platform margin and penetrating sediments of Holocene to Oligocene age that are contained in 17 depositional sequences (A–Q). Two shallow-water sites, Clino and Unda were situated on a extension of the same transect on Great Bahama Bank in a water depth of 10–15 m. With the exception of Unda and Clino, the d13C of the carbonates ranges from +5 per mil in the younger sequences to +1 per mil in the Early Miocene. In each of the sites, the d13C is strongly positively correlated with the percentage of aragonite. As a consequence, the d13C of sequences A through F is strongly correlated, reflecting the decreasing amount of aragonite with increasing depth. In the two platform sites, the d13C is significantly lower in the younger portions of the cores as a result of the influences of meteoric diagenesis during repeated exposure during the Pleistocene. Although the d13C of the individual sequences can be correlated in most instances between the ODP holes, the changes are not related to global changes in the d13C of the oceans which in contrast to the d13C of the platform sediments become isotopically lower towards the present day. Instead variations in the d13C appear to be related to varying mixtures of d13C-rich banktop sediments and pelagic material.本研究针对全新世至早中新世时期台地及台地周缘沉积物钻孔剖面，测定其块状碳酸盐的碳同位素组成（d13C），旨在明确两大科学问题：其一，等效年龄沉积物的碳同位素组成变化是否可实现区域对比；其二，该时期海洋溶解无机碳的碳同位素全球变化是否与上述沉积物的同位素变化存在关联。 其中5个钻孔点位由大洋钻探计划（Ocean Drilling Project, ODP）第166航次完成，剖面距离台地边缘5 km至25 km不等，钻遇全新世至渐新世沉积物，共包含17个沉积层序（A至Q）。 另有两个浅水环境钻孔点位Clino与Unda，设于大巴哈马滩同一剖面的延伸区域，水深为10–15 m。 除Unda与Clino点位外，其余钻孔碳酸盐的d13C值介于年轻层序的+5‰与早中新世层序的+1‰之间。 各钻孔点位的d13C值均与文石（aragonite）含量呈显著正相关。因此，层序A至F的d13C值相关性极强，这反映了文石含量随埋藏深度增加而逐渐降低的规律。 在两个台地钻孔点位中，由于更新世时期反复暴露于大气环境引发的成岩作用影响，岩芯浅层沉积物的d13C值显著偏低。 尽管多数情况下，大洋钻探计划钻孔间的单个层序d13C值可实现对比，但该同位素变化与海洋碳同位素的全球变化并无关联——与台地沉积物的同位素变化趋势相反，海洋溶解无机碳的碳同位素值自渐新世至今呈现逐渐降低的态势。 取而代之的是，沉积物的d13C值变化似乎与富碳同位素的台地顶部沉积物与远洋物质的混合比例差异相关。