TY - JOUR
T1 - Water mass characteristics in the Cenomanian US Western Interior seaway as indicated by stable isotopes of calcareous organisms
AU - Fisher, Cynthia G.
AU - Arthur, Michael A.
N1 - Funding Information:
The authors gratefully acknowledge the support of the National Science Foundation in grant EAR 9304831 to C.F. We also thank the Center for Advanced Scientific Imaging at West Chester University for the use of their scanning electron microscope and Dennis Walizer of the Isotope Biochemistry Laboratory at Pennsylvania State University for assistance with stable isotope analyses. We thank M.B. Hart and Brian Huber for their thoughtful reviews of the manuscript. C.F. would like to thank her mentors of paleoceanographic research, Professors Bill Hay, Don Eicher and Peter Roth.
PY - 2002/12/5
Y1 - 2002/12/5
N2 - Stable oxygen isotopes taken from taxon-specific planktic foraminifera are potential sources of water mass density proxy data and paleoclimate information. Material suitable for stable isotope analysis is uncommon in the middle to late Cenomanian strata of the Western Interior of the USA. However, well preserved material from 17 correlative horizons at five localities across SE Montana and NE Wyoming has provided an opportunity to obtain reliable δ18O data from foraminiferal samples, fine-fraction carbonate (e.g. calcareous nannofossils) and inoceramid prisms. With the exception of inoceramids, which for a benthic organism have stable isotopic compositions resembling surface-dwelling calcareous organisms, these data indicate that the water column was vertically stratified and water masses were anomalously depleted in 18O. Within samples, planktic and benthic foraminifera display strong vertical gradients in δ18O and δ13C of several per mil. The vertical gradient of δ18O ranges from more negative nearer the sea surface to less negative at the sea floor, with a gradient of about 4‰. This implies less saline/warmer waters near the surface with increasingly more saline/colder waters with depth. This isotopic distribution indicates a stably stratified water column at least seasonally. In addition, a persistent vertical carbon isotope gradient of 2-3‰ suggests a strong nutrient gradient that if mixed would result in high surface water fertility. Preservation of high amounts of organic carbon in the sediments corroborates this inference. Although the water column was stratified overall, the nutrients necessary to promote high productivity were clearly renewed at least seasonally. Nutrient renewal could have been achieved by (1) winter cooling of the upper layer allowing deeper seasonal mixing with nutrient-rich deep waters, (2) seasonal shallowing of the pycnocline into the photic zone, (3) entrainment and decomposition of organic matter at the previously proposed frontal convergence, (4) upwelling of nutrient-rich deep waters along the previously proposed oceanic front, or (5) mixing of 'shelf' waters with more oceanic waters across an oceanic front.
AB - Stable oxygen isotopes taken from taxon-specific planktic foraminifera are potential sources of water mass density proxy data and paleoclimate information. Material suitable for stable isotope analysis is uncommon in the middle to late Cenomanian strata of the Western Interior of the USA. However, well preserved material from 17 correlative horizons at five localities across SE Montana and NE Wyoming has provided an opportunity to obtain reliable δ18O data from foraminiferal samples, fine-fraction carbonate (e.g. calcareous nannofossils) and inoceramid prisms. With the exception of inoceramids, which for a benthic organism have stable isotopic compositions resembling surface-dwelling calcareous organisms, these data indicate that the water column was vertically stratified and water masses were anomalously depleted in 18O. Within samples, planktic and benthic foraminifera display strong vertical gradients in δ18O and δ13C of several per mil. The vertical gradient of δ18O ranges from more negative nearer the sea surface to less negative at the sea floor, with a gradient of about 4‰. This implies less saline/warmer waters near the surface with increasingly more saline/colder waters with depth. This isotopic distribution indicates a stably stratified water column at least seasonally. In addition, a persistent vertical carbon isotope gradient of 2-3‰ suggests a strong nutrient gradient that if mixed would result in high surface water fertility. Preservation of high amounts of organic carbon in the sediments corroborates this inference. Although the water column was stratified overall, the nutrients necessary to promote high productivity were clearly renewed at least seasonally. Nutrient renewal could have been achieved by (1) winter cooling of the upper layer allowing deeper seasonal mixing with nutrient-rich deep waters, (2) seasonal shallowing of the pycnocline into the photic zone, (3) entrainment and decomposition of organic matter at the previously proposed frontal convergence, (4) upwelling of nutrient-rich deep waters along the previously proposed oceanic front, or (5) mixing of 'shelf' waters with more oceanic waters across an oceanic front.
UR - http://www.scopus.com/inward/record.url?scp=0037027983&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=0037027983&partnerID=8YFLogxK
U2 - 10.1016/S0031-0182(02)00552-7
DO - 10.1016/S0031-0182(02)00552-7
M3 - Article
AN - SCOPUS:0037027983
SN - 0031-0182
VL - 188
SP - 189
EP - 213
JO - Palaeogeography, Palaeoclimatology, Palaeoecology
JF - Palaeogeography, Palaeoclimatology, Palaeoecology
IS - 3-4
ER -