Late Cenomanian ? Early Turonian Reconstruction of Intermediate and Deep-Water Circulation in the Proto-Indian Ocean



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The late Cenomanian and early Turonian (~96-90 Ma) was an interval characterized by a global warming trend and peak sea surface temperatures during the Mesozoic and Cenozoic eras. The Cenomanian/Turonian boundary (~94 Ma) also 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. A growing body of Nd isotope data is constraining the role of deep-water circulation in organic carbon burial during OAE2. An increase in deep-water ?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. Furthermore, 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 the proto-Indian Ocean Sites to determine the evolution and timing of intermediate- and deep-water Nd isotope values. Deep-water ?Nd(t) values from Site 765 and 766 (3000-4000m paleowater depth) vary coherently between ~-8.5 and ~-5.5 over the interval ~98 to 91 Ma. These sites record a gradual 1.5 epsilon unit increase throughout the study interval. The evolution of intermediate-water ?Nd(t) values in the region were significantly different from the deep-water values. Site 763 ?Nd(t) values ranged from ~-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 ~95 to 93 Ma. Late Cretaceous sea level transgression and extreme global warmth, based on ?18O data, are both contributing factors to the vertical expansion of warm surface waters displayed by the new data. This work shows that the oceanic response to Late Cretaceous warming varied in different regional ocean basins.