In pelagic carbonate sediments, the degree to which the δ13C values of inorganic and organic fractions co‐vary has been used to interpret rates of production, burial and decomposition of organic carbon. This relationship is relatively consistent through time, permitting estimates of organic carbon production and preservation. However, as the majority of pelagic sediments older than 200 Myr have been subducted, carbonate sediments deposited in epeiric seas and platforms are often substituted for pelagic carbonates in analyses of ancient global carbon cycling. There are well‐known pitfalls to using shallow marine carbonate materials, including diagenesis, semi‐isolation of depositional environments and input of different types of sediments with varying inorganic δ13C (δ13Cinorganic) values, which can obscure any global signatures. One method used to assess whether global changes in δ13C are accurately represented by δ13Cinorganic records is to examine variations in the δ13C of co‐occurring organic material (δ13Corganic). If a δ13Corganic record co‐varies with a co‐occurring δ13Cinorganic record, it is argued that the signals must be related to variations in the global carbon cycle. This assumption has been investigated by analysing the isotopic composition of the organic carbon preserved in the uppermost 150 m of periplatform sediments recovered during ODP Leg 166 from the western margin of Great Bahama Bank. The δ13Corganic values measured in this study were compared to previously published δ13Cinorganic records measured on identical samples, thus allowing a study of the correlation between the two records through time. These analyses showed that the correlation coefficient between δ13Cinorganic and δ13Corganic increased from the proximal location (Site 1005, r2 = 0·1), to the distal site (Site 1006, r2 = 0·63). The importance of platform‐derived carbonate and organic material at the proximal location, Site 1005, is reflected in the absence of a co‐variation between inorganic and organic δ13C records, which exhibit no correlation on the platform itself. In contrast, the co‐variance in δ13C values at the basinal location, Site 1006, is explained by a two‐point mixing model, which demonstrates the importance of both pelagic and platform‐derived carbonate and organic carbon in generating the positive correlation between the organic and inorganic δ13C values; this results in a correlation between δ13Cinorganic and δ13Corganic records at Site 1006 that is unrelated to global carbon cycling. Such data question the applicability of using δ13Corganic values to support the ability of δ13Cinorganic values to record global carbon cycling in carbonates recovered from environments where multiple sources of carbonate and organic carbon contribute to the bulk δ13C signal.
The enigma of the Bahamas is that this highly productive carbonate system has existed for at least 100 m.y., building a vast edifice of carbonates, thousands of meters thick, in an essentially nutrientpoor environment. Based on measurements of the insoluble material, the Fe and Mn in the carbonate fraction, and the d 15 N of the sedimentary organic matter, we suggest a paradigm shift in order to explain the formation of the Bahamas and possibly other similar platforms. We propose that the Great Bahama Bank is currently, and may in the past have been, fertilized by atmospheric dust, promoting the fixation of atmospheric N 2 by cyanobacteria. These cyanobacteria provided a source of nitrogen to the rest of the community in this nutrient-poor environment. The fixation of N has imparted a characteristic d 15 N signal and has been responsible, through the drawdown of CO 2 , for initiating the precipitation of carbonate in the shallow waters. This phenomenon might be responsible for the formation of vast amounts of sediments in the oceans, not only within recent times, but throughout geological history, particularly in the early history of the Earth prior to the existence of calcium carbonate-secreting organisms.
We examined the stratigraphy of the top 2100 m of the Great Isaac 1 well, tied it to recent seismic surveys of the area, and correlated it with ODP Site 626, 60 km to the southwest. The succession at Great Isaac includes shallow-water carbonate-evaporite deposits (mid-Cretaceous and older) overlain by deep-water deposits with upward-increasing neritic debris (Late Cretaceous-Tertiary) and capped by bank-margin deposits (Plio-Pleistocene). This succession is interpreted as a restricted carbonate platform that was drowned in the late Albian or Cenomanian, subsided to over 600 m water depth, and was subsequently reintegrated into Great Bahama Bank by westward progradation of the platform. Seismic surveys support this interpretation. The carbonate-evaporite platform appears as high-amplitude parallel reflections, the pelagic chalk as parallel-layered drape over the platform, and the slope as prograding clino forms where coherence of reflections decreases upward with decreasing distance from the platform; the bank margin is a zone of incoherent reflections. In the late Oligocene and Miocene, ODP Site 626 and Great Isaac display almost identical deep-water facies and horizontal time lines, suggesting that both sites lay in the Florida Straits at the same water depth. The older section was not recovered at Site 626, but parallel time lines and similar depositional history are also indicated for the Paleogene and Late Cretaceous: the top of the drowned platform at Great Isaac, which is a high-amplitude seismic reflector there, coincides in depth with the target reflector at Site 626, interpreted on seismostratigraphic grounds as a drowned platform. Borehole stratigraphy and seismic profiles at Great Isaac and Site 626 clearly show that the present margin of Great Bahama Bank in this area is controlled not by tectonics but by platform progradation and that the drowned Cretaceous platform recovered at Great Isaac also underlies the Florida Straits in this area. Both observations strongly support the hypothesis that until the mid-Cretaceous, Florida and the Bahamas were welded into a single vast platform, the megabank.
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