Abstract:
The Cenomanian /Turonian (C/T) boundary ( approximately 94 Ma) was a time of peak sea surface temperatures during the Mesozoic and Cenozoic eras, and coincided with widespread burial of organic carbon (Oceanic Anoxic Event 2 - OAE2). Several factors likely promoted organic carbon burial including increased nutrient input, diminished seafloor oxygen levels, density stratification, enhanced upwelling, and sluggish deep-water circulation. Nd isotope data is constraining the role of deep-water circulation in organic carbon burial during OAE2. An increase in deep-water epsilon (sub Nd) (t) values across the C/T in the tropical Atlantic is interpreted to reflect a change in deep-water circulation that may have led to higher sea surface nutrient levels, suggesting deep-water circulation was not sluggish. But assessing the broader role of deep-water circulation in organic carbon burial during OAE2 requires data from other ocean basins. Previously published low-resolution Nd isotope data from proto-Indian Ocean ODP Sites 763, 765, 766, and 1138 suggested a change during the mid-Cretaceous that may have coincided with the C/T. Here we present new high-resolution data from these sites to determine the evolution and timing of intermediate- and deep-water Nd isotope values. Deep-water epsilon (sub Nd) (t) values from Site 765 and 766 (3000-4000m paleowater depth) vary coherently between approximately -8.5 and approximately -5.5 over the interval approximately 98 to 91 Ma. Both sites record a two-step transient approximately 1.5 epsilon unit increase across the C/T interval. Intermediate-water epsilon (sub Nd) (t) values in the region were significantly different from the deep-water values. Site 763 epsilon (sub Nd) (t) values ranged from approximately -10.5 to -11.1 from 95.8 to 92.4 Ma, then increased to -7.1 by 89.8Ma. In the western portion of the basin, intermediate-water values recorded at Site 1138 decreased from -4.4 to -6.7 from approximately 95 to 93 Ma. The different intermediate- and deep-water epsilon (sub Nd) (t) values suggest that different water masses were circulating in the relatively small basin sufficiently vigorously to retain their isotopic signatures. This interpretation is consistent with the reconstruction of circulation in the tropical Atlantic, implying that deep-water stagnation was not a key factor in organic carbon burial during OAE2.