Biogeochemistry of Late Paleozoic North American brachiopods and secular variation of seawater composition |
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Authors: | Uwe Brand |
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Institution: | (1) Department of Geological Sciences, Brock University, St. Catharines, Ontario, L2S3A1, Canada |
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Abstract: | Sr/Ca ratios in modern brachiopod shells reflect variations in ambient seawater, whereas their Na contents show no relationship
with water depth or habitat. Their Mn and Fe contents are controlled, in part, by leaching of these elements from oxide coatings
or the low input/sedimentation rate of detrital material into depositional areas such as Quatsino Sound.
For most Carboniferous brachiopods from North America, the Mn and Fe contents are similar to those recorded by their Recent
counterparts. The high Mn and Fe contents in the brachiopods from shales suggest several possibilities for these levels. One
possibility is the leaching of Mn and Fe from oxide coatings/matrix which was not completely removed in the cleaning process,
or the high levels in part reflect unusual depositional conditions (some degree of anoxia) for the local shaly environments.
The Sr/Ca ratio of brachiopods and, by inference, complementary seawater, did not vary significantly during the Carboniferous.
The Sr/Ca minimum observed in brachiopods of Mississippian age coincides with a dip in the 87Sr/86Sr curve and correlates with the Hercynian orogeny. This is attributed to the cycling of seawater through mid-ocean ridge
basalts, and postulated exchange reactions account for variation in the composition of seawater-Ca. The unidirectional trend
of heavier δ13C values from the Devonian to the Permian is intricately coupled with the evolution of the terrestrial biomass. In addition
to expansion of terrestrial plants, burial of reduced carbon in the form of coal (organic matter) contributed to the observed
shift. The start of the Permo-Pennsylvanian glaciation is marked by a negative excursion of the secular carbon trend, which
is linked to weathering of reduced carbon and its return to the oceanic reservoir with its oxidized carbon. The oxygen isotope
values reflect the unidirectional trend towards higher values of the carbon data with decreasing geologic age. Negative excursions
of the trend may be related to extensive weathering of terrestrial and submarine rocks, whereas positive excursions may be
related to hydrothermal alteration of submarine rocks and dehydration of oceanic crust during times of active sea-floor spreading.
Oxygen-calculated water temperatures of unaltered brachiopod material are unrealistically high for all of the Devonian, and
the Chesterian-Meramecian, Desmoinesian-Missourian, and Artinskian Epochs. During these times maximum water temperatures of
42° to 56°C are well above the thermal threshold of protein denaturation. This process, which is lethal to most higher organisms,
demands an adjustment in oxygen of -2.5%. for samples older than Missourian, and of -1.250%. for samples spanning the Missourian-Artinskian
interval. With these adjustments and salinity considerations made prior to calculations, water temperatures become reasonable
for the Late Paleozoic epeiric, tropical seas of North America. |
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Keywords: | Brachiopod biogeochemistry diagenesis secular isotopic variations carbon isotopes oxygen isotopes Sr/Ca temperature Recent Late Paleozoic |
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