Browsing by Subject "Nd"
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Item Late Cenomanian ? Early Turonian Reconstruction of Intermediate and Deep-Water Circulation in the Proto-Indian Ocean(2013-09-24) Tilghman, David SThe 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.Item Lead Isotopic Variations of the Pacific and Implications for Paleogene Water Mass Composition(2013-11-13) Subt, CristinaTo understand the effects changes in the meridional overturning circulation (MOC) played on past climate we need to determine where convection occurred. The late Cretaceous and early Paleogene interval (~75 to 35 Ma) was characterized by low meridional temperature gradients. Nd isotopes indicate high-latitude convection in the Pacific Ocean during this interval, with deep waters mixing in the tropical Pacific. Here we investigate the evolution of the Pb isotopic composition of water masses in the Pacific to constrain variability in weathering inputs into the inferred convection regions from ~75 to 35 Ma. We generated ^(206),^(207),^(208)Pb/^(204)Pb isotope records for North Pacific DSDP and ODP Sites 192, 464, and 883, and South Pacific DSDP Sites 323, 463, and 596. South Pacific deep waters increasing in contributions. Pb composition in the subtropical South Pacific also suggests a stronger influence of dust dissolution whereas the Nd composition was dominated by water mass composition. North Pacific Pb values may also have reflected shifting sources during the EECO. The Pb and Nd composition at Site 883 shows a short radiogenic excursion in Pb values ~40 Ma, possibly cased by a short period of strengthened North Pacific Deep Water (NPDW) influence on the isotopic composition during the Mid-Eocene Climatic Optimum (MECO). ^(206)Pb/2^(04)Pb trends differ from ^(207),^(208)Pb/^(204)Pb values, which typically show similar trends to each other?resulting from the mixing of multiple Pb sources. North Pacific sites typically exhibit relatively lower Pb compositions than South Pacific, and evidence from detrital analyses suggests sources of Pb to North Pacific sites received strong contributions of continental dust. Pb and Nd are coupled in North Pacific sites between ~62 and 50 Ma, as well as in the Southern Ocean throughout the study period, indicating the Pb composition in this region was influenced by the composition of advected deep water masses. North Pacific sites also show decoupling of Pb and Nd ~50 Ma, during the Early Eocene Climatic Optimum (EECO), which may be indicative of more distal