科研之友
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摘要
Climate variability is driven by a complex interplay of global-scale processes and our understanding of them depends on sufficient temporal resolution of the geologic records and their precise inter-regional correlation, which in most cases cannot be obtained with biostratigraphic methods alone. Chemostratigraphic correlation based on bulk sediment carbon isotopes is increasingly used to facilitate high-resolution correlation over large distances, but complications arise from a multitude of possible influences from local differences in biological, diagenetic and physico-chemical factors on individual delta C-13 records that can mask the global signal. To better assess the global versus local contribution in a delta C-13 record it is necessary to compare numerous isotopic records on a global scale. As a contribution to this objective, this paper reviews bulk sediment delta C-13(carb), records from the Late Cretaceous in order to identify differences and similarities in secular delta C-13 trends that help establish a global reference delta C-13 record for this period. The study presents a global-scale comparison of twenty delta C-13 records from sections representing various palaeo-latitudes in both hemispheres and different oceanic settings from the Boreal, Tethys, Western Interior, Indian Ocean and Pacific Ocean, and with various diagenetic overprinting. The isotopic patterns are correlated based on independent dating with biostratigraphic and paleomagnetic data and reveal good agreement of the major isotope events despite offsets in absolute delta C-13 values and variation in amplitude between the sites. These differences reflect the varying local influences e.g. from depositional settings, bottom water age and diagenetic history, whereas the concordant patterns in delta C-13 shifts might represent delta C-13 fluctuations in the global seawater dissolved inorganic carbon. The latter is modulated by variations in organic matter burial relative to re-mineralization, in the global-scale formation of authigenic carbonate, and in partitioning of carbon between organic carbon and carbonate sinks. These variations are mainly controlled by changes in climate and eustasy. Additionally, some globally synchronous shifts in the bulk delta C-13(carb) records could result from parallel variation in the contribution of authigenic carbonate to the sediment. Formation of these cements through biologically mediated early diagenetic processes is related to availability of oxygen and organic material and, thus, can be globally synchronized by fluctuations in eustasy, atmospheric and oceanic oxygen levels or in large-scale oceanic circulation. Because the influence of early diagenetic cements on the bulk delta(13)Cc(atrb) signal can, but need not be synchronized, chemostratigraphy should not be used as a stand-alone method for trans-continental correlation, and especially minor isotopic shifts have to be interpreted with utmost care. Nevertheless, the observed consistency of the delta C-13 correlations confirms global scale applicability of bulk sediment delta C-13 chemostratigraphy for the Late Cretaceous, including sediments that underwent lithification and burial diagenesis such as the sediments from the Himalayan and Alpine sections. %26lt;br%26gt;Limitations arise from increased uncertainties (1) in sediments with very low carbonate content, (2) from larger delta C-13 variability in sediments from very shallow marine environments, (3) from unrecognized hiatuses or strong changes in sedimentation rates, and (4) in sections with short stratigraphic coverage or with few biostratigraphic marker horizons. %26lt;br%26gt;The combination of chemostratigraphy with biostratigraphy and magnetostratigraphy substantially increases the precision and temporal resolution of inter-regional correlations and helps overcome problems that arise from differences in biostratigraphic schemes, facies or provincialism of key fossils. By using an iterative approach to stepwise increase precision of the correlations, isochroneity of first and last occurrences of marker species versus chemostratigraphy is tested, which helps to improve biostratigraphic zonations, to assess zonal boundary ages and to identify useful criteria for defining Late Cretaceous stage boundaries, many of which are still not formally defined. The presented correlations indicate a consistent position for most planktic foraminifer zonal boundaries relative to corresponding isotope shifts during the mid-Cretaceous sea-level high, whereas diachroneity appears to be more pronounced during the Late Campanian and Maastrichtian global sea-level fall. A similar pattern is observed for trans-continental consistency in the delta C-13 shifts. Graphic correlation of isotopic shifts, magnetostratigraphic and biostratigraphic events among the compared sections is used to detect hiatuses or relative changes sediment accumulation rates and visualizes consistency or offsets of individual biostratigraphic markers relative to chemo- and magnetostratigraphy. Finally, an attempt of a global average delta C-13 stack is presented for the Turonian through Maastrichtian.
- 出版日期2013-11
