Evolution of neodymium isotopic signature of seawater during the Late Cretaceous: Implications for intermediate and deep circulation
Evolution of neodymium isotopic signature of seawater during the Late Cretaceous: Implications for intermediate and deep circulation
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DOI：
10.1016/j.gr.2015.08.005
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发表时间：
2016-08
影响因子：
6.1
通讯作者：
Mathieu Moiroud;E. Pucéat;Y. Donnadieu;G. Bayon;M. Guiraud;S. Voigt;J. Deconinck;F. Monna
中科院分区：
文献类型：
--
作者：
Mathieu Moiroud;E. Pucéat;Y. Donnadieu;G. Bayon;M. Guiraud;S. Voigt;J. Deconinck;F. Monna
近50年来，εNd被广泛应用于过去海洋环流的示踪，近10年来更被广泛应用于研究白垩纪海洋环流。尽管有越来越多的数据，但这一时期的流通模式仍然不清楚。特别是，确定深水块及其在不同洋盆内的空间延伸受到很大的限制。本研究中，我们提出了新的深水ε Nd数据推断的Nd同位素组成的鱼类遗骸和Fe-Mn羟基氧化物涂层有孔虫测试，沿着新的ε Nd数据的残余从DSDP站点回收的（部分碎屑）部分152（Maud Rise）、258（Naturaliste Plateau）、323（Bellinghausen深海平原）和ODP站点690（Maud Rise）和700（东格鲁吉亚盆地，南大西洋）。152和690站位沉积物中存在丰富的自生矿物，这可能解释了残渣态光谱中稀土元素的富集和残渣态ε Nd的演化，反映了这两个站位底部沃茨的演化。结果表明，258号和700号站点从晚土伦期到桑托期的底层水ε Nd值之间存在着密切的对应关系。由于深水Nd同位素值在这两个网站也类似于其他原印度网站，我们建议存在一个共同的中间深水质量早在白垩纪中期。水团将从南大西洋的中部延伸到原始印度洋的东部，越过凯尔盖朗高原。此外，从格兰德河上升-沃尔维斯岭复合体（站点700和530）的南部和北部的数据是无法区分的从Turonian到Campanian，这表明一个共同的水团，因为Turonian至少。这一观点得到了重建的格兰德河上升沃尔维斯岭复合体在Turonian的支持，突出了一个很可能存在的深刻的突破之间的格兰德河上升和原沃尔维斯岭在那个时候。因此，尽管存在潜在的海洋屏障，但早在土伦期，不同的南部盆地之间就可能存在深水环流。新海水和残留ε Nd数据的对比表明，在马斯特里赫特期和古新世，从北大西洋到太平洋的中间沃茨通过加勒比海道向西环流。这种向西的环流减少了太平洋水对大西洋的影响，可能是80 Ma后北大西洋更均匀、辐射成因更少的ε Nd值的原因。此外，我们的数据文件在马斯特里赫特期和古新世，这是在北太平洋更为明显的几个海洋盆地观察到的增加趋势。虽然这种增加的原因尚不清楚，但可能是北方太平洋上层海洋沃茨的放射性物质增加所致。这些沃茨下沉到一定深度后，可能通过深水交换，在某种程度上将更多的放射性成因特征重新分配到其他海洋盆地。
Neodymium isotopic compositions (εNd) have been largely used for the last fifty years as a tracer of past ocean circulation, and more intensively during the last decade to investigate ocean circulation during the Cretaceous period. Despite a growing set of data, circulation patterns still remain unclear during this period. In particular, the identification of the deep-water masses and their spatial extension within the different oceanic basins are poorly constrained. In this study we present new deep-water εNddata inferred from the Nd isotope composition of fish remains and Fe–Mn oxyhydroxide coatings on foraminifera tests, along with new εNddata of residual (partly detrital) fraction recovered from DSDP Sites 152 (Nicaraguan Rise), 258 (Naturaliste Plateau), 323 (Bellinghausen Abyssal Plain), and ODP Sites 690 (Maud Rise) and 700 (East Georgia Basin, South Atlantic). The presence of abundant authigenic minerals in the sediments at Sites 152 and 690 detected by XRD analyses may explain both middle rare earth element enrichments in the spectra of the residual fraction and the evolution of residual fraction εNdthat mirror that of the bottom waters at the two sites. The results point towards a close correspondence between the bottom water εNdvalues of Sites 258 and 700 from the late Turonian to the Santonian. Since the deep-water Nd isotope values at these two sites are also similar to those at other proto-Indian sites, we propose the existence of a common intermediate to deep-water mass as early as the mid-Cretaceous. The water mass would have extended from the central part of the South Atlantic to the eastern part of proto-Indian ocean sites, beyond the Kerguelen Plateau. Furthermore, data from south and north of the Rio Grande Rise–Walvis Ridge complex (Sites 700 and 530) are indistinguishable from the Turonian to Campanian, suggesting a common water mass since the Turonian at least. This view is supported by a reconstruction of the Rio Grande Rise–Walvis Ridge complex during the Turonian, highlighting the likely existence of a deep breach between the Rio Grande Rise and the proto-Walvis Ridge at that time. Thus deep-water circulation may have been possible between the different austral basins as early as the Turonian, despite the presence of potential oceanic barriers. Comparison of new seawater and residue εNddata on Nicaraguan Rise suggests a westward circulation of intermediate waters through the Caribbean Seaway during the Maastrichtian and Palaeocene from the North Atlantic to the Pacific. This westward circulation reduced the Pacific water influence in the Atlantic, and was likely responsible for more uniform, less radiogenic εNdvalues in the North Atlantic after 80 Ma. Additionally, our data document an increasing trend observed in several oceanic basins during the Maastrichtian and the Palaeocene, which is more pronounced in the North Pacific. Although the origin of this increase still remains unclear, it might be explained by an increase in the contribution of radiogenic material to upper ocean waters in the northern Pacific. By sinking to depth, these waters may have redistributed to some extent more radiogenic signatures to other ocean basins through deep-water exchanges.