Diagenetic Alteration of Carbonates

Studies of diagenesis, the processes of fluid-rock interaction which modify the chemistry of carbonate, are an important component of my past and present research program. Carbonates are highly susceptible to alteration, including changes in mineralogy and chemistry. As such, to ensure that records recovered from ancient materials accurately reflect their original chemistries, it is essential to determine the extent of this diagenetic alteration if the original chemical record is to be resolved. Although our understanding of diagenetic processes is quite advanced, the challenges of our new high resolution research require that this remain an area of continuing commitment.

Long term Variations in Ocean Chemistry

The ocean is an integrating system whose chemistry reflects the changing dominance of earth surface processes including volcanism, mountain building, seafloor-spreading, and weathering. In particular, I have concentrated on deciphering the secular variation of carbon and oxygen isotopes of marine carbonates as records of first-order changes in geologic process that have occurred during the last billion years. One significant finding of this research is the documentation of an abrupt transition in the oxygen isotope composition of the ocean which occurred approximately 360 million years ago. We interpret this chemical event to reflect a shift from dominantly low temperature weathering of crustal rock, to an increase in the hydrothermal flux to the ocean associated with high temperature alteration of the crust at spreading ridges. New areas of research in paleocean chemistry are developing multi-tracer methods that enable us to also determine primary changes in the minor element chemistry of the ocean.

Seasonality of Continental Interiors During Climate Transitions

The distribution of warm-climate plants and animals to high paleolatitudes has been used to argue for decreased temperature seasonality of continents during times of global warming (75 to 40 million years ago). Such interpretations, however, are in conflict with results of computer simulations of climate which predict that continental interiors should retain, or even expand, the range of seasonal temperature change. Several concurrent projects are underway to provide quantitative measurements of temperature seasonality for continental sites. For example, our unique microsampling techniques permit direct analysis of the accretionary growth structure present in clams and fish ear bones (otoliths) with a time resolution of weekly to submonthly intervals. This is an exciting new direction in research and our initial findings are receiving international recognition. Future research is expanding the scope of such studies to include evaluation of climate associated with human habitational sites (in collaboration with anthropologists) as well as high resolution studies for the last ten thousand years during the transition in climate following the last glacial episode in North America.

Long term climate and seasonality of Antarctic coastal marine sites: late Cretaceous and Paleogene

Overview

The objectives of our pilot research study were threefold: 1) examination of the long term trend in d18O of bivalve carbonate to identify stratigraphic intervals for intensive future studies of temperature seasonality; 2) development of elemental proxies for temperature and salinity to enable interpretation of d18O records relative to both temperature and salinity; and lastly, 3) to examine intra-annual records of d18O bivalve carbonate to reconstruct temperature seasonality and to extract maximum sea surface summer temperatures for coastal regions of Antarctica during the Paleogene. Progress has been made in all three areas although preliminary research has focused on development of the elemental proxies for temperature and salinity. Future work will continue to refine these empirical calibrations and apply them to fossil material to reconstruct shallow water environmental conditions present during the Paleogene.

Long-term Trend in d18O of Marine Bivalves

Our pilot study focused on the bivalve genus Cucullaea which is present throughout the stratigraphic succession of the La Meseta Fm of Seymour Island. To obtain average isotopic values, we sampled the first 3 years of growth by drilling each shell at the umbo with a 0.5 mm burr. All specimens are excellently preserved retaining their primary aragonitic mineralogy. Sampling concentrated on the lowermost Unit 2 (late Paleocene) and the uppermost Unit 7 (middle Eocene to late Eocene?). Additionally, 2 specimens were examined in Unit 5 to provide preliminary data throughout the stratigraphic succession. Significantly, the range of isotopic composition observed in this study differ markedly from those previously documented by Gazdzicki et al. 1992 which observed a total range from 1.35 to 2.06‰ d18O. We observed a total range from -0.2 to 1.5 ‰ with distinct shifts stratigraphically (see attached figure). Additionally, we observe a temporal trend in d18O: Unit 2 (average = 0.4 ‰) decreases in Unit 5 (-0.2 ‰) followed by a general increase into Unit 7 (1.5 ‰). We have also examined bivalves recovered from the high latitude Site 270 (late Oligocene to early Miocene) which yields increasingly more positive d18O values (3.0 ‰) and from modern Laternula collected from McMurdo Sound (4.5 ‰). Assuming constant marine salinity and composition of -1 ‰ SMOW, the span of d18O values corresponds to an approximate shift of 7-8 °C. Absolute values estimate a maximum temperature of: 14 °C for Unit 2; 17 °C for Unit 5; and 9 to 11 °C for Unit 7. Similar estimation for Site 270 yields 2-5 °C for late Oligocene and early Miocene surface waters. Data obtained from the La Meseta Fm. bivalves are compatible with estimates for surface water temperatures based on shallow dwelling planktic foraminifers (Zachos et al. 1994). From planktic foraminifer d18O values they estimate 11 °C for the late Paleocene, 14 °C for the early Eocene, and 11 °C for the early middle Eocene with a decrease to 3-6 °C for the late Eocene. If after more rigorous sampling this temporal trend remains, not only will it be possible to provide estimates of shallow water temperature maxima, but it also may be possible to utilize the thermal record for correlation with open ocean planktic foraminfer stratigraphies.

Elemental Proxies of Paleosalinity and Paleotemperature

Our studies are focused on the elemental and isotopic composition of the marine mussel Mytilus, which is cosmopolitan in its distribution, and whose geologic record extends from the Eocene to Recent. Sequential microanalysis of annual growth bands provided a time series with weekly resolution. To decipher the competing effects of temperature versus salinity variation, we directly paired elemental and isotopic measures on recovered microsamples. Following the pioneering work of Dodd (1969) and Lorens and Bender (1980), we have refined the relationship between temperature and Mg content in the calcitic portion of Mytilus such that Mg content serves as an independent proxy of growth temperature. Similar examination of Sr content suggests that variations in composition are correlated to changes in salinity . Through a comparison of d13C and Sr trends, in combination with a Mg/Ca paleothermometer, it may be possible to uniquely determine past changes in paleosalinity and paleotemperature of coastal marine settings.

Uplift and Exposure History of the Cote D'Ivoire-Ghana Transform Margin: Geochemistry of Porefilling and Fracture Vein Calcites

Reefal and peri-platform carbonates of late Albian to Turonian-Cenomanian age were recovered from Ocean Drilling Program Sites 959, 960 and 962 (ODP Leg 159) which were drilled on the Côte D'Ivoire-Ghana Transform Margin. These deposits thicken and coarsen from the Deep Ivorian Basin southward toward the marginal ridge and are intercalated with pelagic micrites containing abundant planktonic foraminifers. The stratigraphic relations along with the nature of the skeletal grains and intraclasts in the carbonates suggest a depositional setting of a shallow carbonate platform which rimmed an emergent metamorphic and sedimentary terrane that served as a coeval source of clastic and carbonate components. Carbonate-rich horizons are characterized by dissolution of aragonitic bioclasts, recrystallization of micrite to microspar, and infilling of primary and secondary porosity by equant to prismatic low-Mg calcite cements. Preservation of grain morphologies, as uncompacted micrite rinds suggests early dissolution and concomitant cementation at the sediment-surface or during shallow burial. Late stage fracturing of these units during syn- and post-transform deformation is marked by extensive mineralization by quartz, barite, clay, and calcite vein fills. Based on paragenetic relations and isotopic composition, multiple stages of fracturing and mineralization are identified. All cements and fracture vein calcites have d13C values significantly more negative than primary marine compositions, suggesting a dominant role of organic matter decomposition in controlling the isotopic composition of diagenetic fluids. The oxygen isotopic composition of cements and vein fills range form -1‰ to -10‰ d18O. Of these, the first stage is interpreted as forming during progressive subsidence under increasing temperature conditions. A late fracturing event is recorded by calcite whose composition ranges from -8‰ to -10 ‰. Assuming a reasonable range of interstitial water compositions, -5‰ to +2 ‰ SMOW, a maximum emplacement temperature of less than 90 °C is estimated for late fracture vein calcites. This stage of fracturing and calcite mineralization, which reflects the highest temperature event of calcite formation, occurs in units as young as early Eocene in age and requires precipitational temperatures in excess of those expected from typical burial diagenesis. In light of the high temperature of emplacement, this last stage of deformation is interpreted as forming during the passing of the spreading ridge along the Côte D'Ivoire-Ghana Transform Margin during early Eocene time, a time much later than predicted based upon previous studies.