Bathymetry and productivity of the southern Great Basin seaway, Nevada, USA: An evaluation of isotope and trace element chemistry in mid-Carboniferous and modern brachiopods

摘要

The stable isotope and trace element chemistry of brachiopods from stratigraphically, paleontologically, and geochemically three coeval intervals about the mid-Carboniferous boundary in the Bird Spring Formation were evaluated for their potential of serving as proxies of bathymetry and productivity of the southern Great Basin seaway, an eastern arm of the Panthalassa Ocean. Oxygen isotopes delineate both spatial and temporal differences in values of brachiopods from the three coeval mid-Carboniferous sections. Based on pristine brachiopod delta O-18 values and supported by paleoecological information, water depth increased from Apex to Kane Springs Wash East to Arrow Canyon. In addition, there is also an increase in values from the latest Mississippian to the earliest Pennsylvanian suggesting a water temperature (cooling), a water depth (increase), and/or change in both of the habitat of the penecontemporaneous brachiopods. Analogous to values and trends observed in modem brachiopods, the Fe/Mn ratio of fossil counterparts may be a potential proxy of seawater productivity related to the micronutrient iron. Overall, the high ratios of Fe/Mn suggest highly productive waters for the southern Great Basin seaway, with somewhat reduced productivity during the earliest Pennsylvanian. In contrast, the carbon isotope values of the coeval material are relatively invariant with respect to productivity; trends similar to observations on modern carbonate allochems. Based on habitat and paleogeography, the Antler Orogenic Highlands west of the foreland basin are considered a potential source for the micronutrient iron enrichment of the carbonate platform shelf-edge environment. It is further postulated that upwelling currents, in part, which weakened from the latest Mississippian through to the earliest Pennsylvanian, may have transported the micronutrient iron onto the shelf/basin environments of the Bird Spring Formation of the southern Great Basin. (C) 2007 Elsevier B.V. All rights reserved.