Carbonate mineral cycles in ramp slope sediment: Eastern Gulf of Mexico

Abstract

Carbonate mineral cycles, controlled by Pleistocene climatic rhythms, have been discovered in a 11.6 m‐long‐piston core from the west Florida carbonate ramp slope at 890 m of water depth in the eastern Gulf of Mexico. Relatively high concentrations of aragonite, magnesian calcite, and dolomite, as well as insoluble residues, total organics, and mud‐sized sediment, all correlate with glacial periods defined by oxygen isotope stratigraphy. The aragonite is a low‐strontium variety derived from pteropods, and there is no evidence that concentration variations are controlled by dissolution. We propose that these mineral and textural cycles are the product of a combination of increased biological productivity and dilution by fine‐grained terrigenous mud during glacial episodes. Aragonite cycles in Bahamian periplatform slope sediment have also been correlated with climatic change. There the aragonite is a high‐strontium variety derived from shallow‐water calcareous algae, with relatively high concentrations correlated with high sea levels during interglacial periods. The carbonate mineral cycles along the west Florida ramp slope are 180° out of phase with Bahamian examples and also lack their marked asymmetry. It is now apparent that for deepwater, continental margin carbonates there are two types of carbonate mineral cyclicity: one that is “pelagic driven” during glacial periods, and one that is “platform driven” during interglacials. These contrasts emphasize the differences in deep water cyclic sedimentation on carbonate ramps versus rimmed carbonate platforms that are controlled by physiography, sediment source, oceanographic setting and proximity to terrigenous input. The productivity‐dilution model proposed here may be a valuable modern analog for ancient carbonate sequences deposited in epicontinental seas or on ramps attached to continental margins.

Facies and faunal assemblage changes in response to the Holocene transgression in the Lagoon of Mayotte (Comoro Archipelago, SW Indian Ocean)

“…The results of this study confirm faunal changes during a sea-level rise observed in present-day and fossil mixed terrigenous-carbonate lagoons and/or rimmed carbonate platforms (Kuhn 1984;Gardulski et al 1986;Belperio and Searle 1988;Crevello 1991;Peerdeman and Davies 1993;Blomeier and Reijmer 1999;Hillgärtner 1999;Strasser et al 1999;Gischler 2003). An almost similar facies succession in response to the Holocene sea-level rise, like for the proximal-and mid-lagoonal settings within the lagoon of Mayotte, is found in the North-lagoon of Bermuda (Kuhn 1984).…”

Sedimentological Response of an Outer-Shelf, Upper-Slope Sequence to Rapid Changes in Pleistocene Eustatic Sea Level: Hole 820A, Northeastern Australian Margin

“…(3) a drastic change in grain size and carbonate content at the transition from lowstand to early transgression, with the abundance of sand decreasing by 50% and carbonate content decreasing by 45%; (4) sand-sized constituents only containing planktonic and benthic foraminifers and fragments of pteropods; (5) an increase in the abundance of terrigenous components of the mud fraction with terrigenous clays, quartz, and hematite as dominant mineralogical phases; (6) a stable, single-domain remanence with a significant contribution from ultrafine, superparamagnetic grains, defined by Barton et al (this volume) as marking the sharp peak of maximum susceptibility just after the lowstand in sea level (heaviest δ 18 θ value) and interpreted to define the derivation from either the continent or the exposed platform; and (7) minor abundances of aragonite having a Ar/Ca ratio of 0.25 and defining the early transgression, but increasing to approximately 0.55 during the late transgression. A similar carbonate trend also is seen in the sediments of the eastern Gulf of Mexico (Gardulski et al, 1986) and has been attributed to pelagic-driven sedimentation during glacial periods, switiching over to platform-driven during interglacial periods. This is different from the process-response in the Bahamas, where variations in concentration of aragonite have been explained as climatic related dissolution (Droxler et al, 1983).…”

“…Comprehensive studies of the textural and compositional characteristics of carbonate-dominated shelf and slope sediments have already been performed on the continental margins of, for instance, the eastern Gulf of Mexico and the Bahamas. These studies have focused on the signature of Pleistocene sea-level lowstands and highstands, seen in slope sediments of mixed carbonate-siliciclastic systems having carbonate ramps (Gardulski et al, 1986;Roof et al, 1991) and carbonate-dominated margins having rimmed platforms (Kier and Pilkey, 1971;Boardman and Neumann, 1984;Boardman et al, 1986;Droxler et al, 1983). These studies have shown (1) that textural and compositional variation of periplatform sediments encapsulate a history of response of carbonate bank-tops to fluctuations in sea level (Boardman and Neumann, 1984;Boardman et al, 1986), although others have demonstrated the importance of variable surface productivity (Gardulski et al, 1986) and dissolution of the unstable aragonite phase (Droxler, 1985) in the deduction of sea level from periplatform deposits, and (2) that the recognition of the depositional environment (rimmed platform or ramp attached to a continent) is critical for interpreting cyclic slope sedimentation and is the key to the study of ancient cyclic carbonate sequences (Gardulski et al, 1986).…”

“…Such rates are explained by exposure of the platform during the period of relative decrease in sea level that caused platform production and shedding to cease (Kier and Pilkey, 1971;Sarg, 1988). This is different from mixed carbonatesiliciclastic systems in the eastern Gulf of Mexico, where regression and lowstand deposits are reported to reflect the influx of terrigenous mud from the continent during periods of low sea level, thus diluting the pelagic components and reducing relative sand concentration (Gardulski et al, 1986;Roof et al, 1991). Our data show no evidence for dilution; on the contrary, the percentage of carbonate-rich sand increases throughout the regression to maximum values during the lowstand, with a high carbonate peak pre-dating the sea-level lowstand.…”

Quaternary CaCO3 Input and Preservation within Antarctic Intermediate Water Mineralogic and Isotopic Results from Holes 818B and 817A, Townsville Trough (Northeast Australian Margin)

“…Studies of modern seafloor sediments have shown that pteropod fragments increase below the aragonite lysocline because partial dissolution of the thin pteropod shells makes them more susceptible to breakage (Chen, 1968;Almogi-Labin et al, 1986). Percent of whole pteropods was used rather than simply the abundance of pteropods because changes in pteropod abundance may largely be related to pteropod productivity and bear little relationship to preservation (Gardulski et al, 1986). We were not able to completely avoid the complication of variable pteropod productivity, however, because we calculated a percent of whole pteropods value of 0% if the sample contained no pteropods.…”