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A closer look at limestones in Valdehuesa, Leon, Spain.

Heather M. Stoll
Assistant Professor

Williams College Department of Geosciences
hstoll@williams.edu

Tel: (413) 597 - 4541

Ph.D. in Geochemistry, Princeton University, 1998
B.A. in Geology,Williams College, 1994

My research interests span timescales from the Mesozoic through the Quaternary, pursuing questions as diverse as the origin of rapid sea level changes in the Cretaceous to how glacial/interglacial temperature changes are recorded in carbonate sediments. My recent work investigates the history of the oceans and climate (1-6) and their complex interactions as archived in the sediment record, although some of my earlier work focused on modern surficial processes (7) and geochemistry of ancient volcanic rocks (8) . These varied projects have in common the use of the chemistry of rocks, sediments, and waters as a tool to unravel the earth's history and better understand its continuing evolution.

In the last several years, I have pioneered the development of a new tool for gathering information about past climates and ocean productivity from marine geochemical records. The Sr/Ca ratio of coccoliths serves as an indicator of the productivity of marine coccolithophorid algae. Since the coccolithophorids have a widespread distribution in the ocean and have existed since the Early Mesozoic, the calcite they preserve in the sediments(see SEM photo of coccoliths in sediments) represents a potentially enormous record which to date has not been utilized in minor element studies. Controlled culturing experiments, and analyses of plankton samples from oceanographic research cruises, sediment traps from the Indian Ocean, and surface sediments from several oceanographic settings, demonstrated that coccolith Sr/Ca reflects coccolithophorid productivity. Indicators such as coccolith Sr/Ca can be applied with greatest confidence where the underlying mechanisms are well understood. I have used numerical models of surface enrichment effects during crystal growth to demonstrate that large coccolith Sr/Ca variations cannot arise from purely crystal-kinetic mechanisms and must reflect biological control over uptake and transport of Sr and Ca ions into the cell where coccoliths are made.

To enhance the utility of this new indicator, I developed new methods to separate out monospecific fractions of coccoliths from sediments using differential settling velocities of different shapes and sizes of coccoliths of different species. Previous geochemical studies (of stable isotopes) in coccoliths had been limited by the inability to individually pick the small (2-10 mm) coccoliths as is routinely done with other microfossils; this is important since different species may have different biological controls over their chemistry as well as different responses to changing environmental conditions. In the last year, I have applied these new separation methods to measure Sr/Ca variation in several species across upwelling productivity gradients. This is the first indication that species-level productivity response can be identified, and is likely to be useful in enhancing global carbon cycle models to include accurate ecological responses of primary producers in predictions of future ocean uptake of anthropogenic CO2 as well as investigating paleoproductivity events.

Currently, we are using coccolith Sr/Ca variations to study productivity variations during the Cretaceous, Paleocene, and Pleistocene.

The Cretaceous Period is characterized by an apparent paradox between the sequence stratigraphic record of rapid global sea level changes and evidence that the warm greenhouse climate precluded the development of ice sheets, the only accepted mechanism for such sea level changes. Central questions are whether ice sheets can periodically exist during extreme greenhouse warmth, what sort of temporal and spatial variability characterizes such climates, and how sensitive the climate is to perturbations in the carbon cycle. In the course of my doctoral program, fieldwork in the Betic Cordillera in Spain and the Italian Apennines (see field photo) provided excellent deep sea sediment records for the mid-Cretaceous. From these sections, I have produced very high resolution records of d13C and d18O of carbonates. Correlation of large positive d18O excursions (implying colder climates and/or greater ice volume) with global sea level regressions identified by sequence stratigraphy may indicate that episodes of polar ice accumulation periodically interrupted the warm Cretaceous greenhouse climate.

My work on older carbonates necessitates a clear understanding of how the chemistry of these carbonates may be altered diageneically during burial. For the Cretaceous studies, I considered the likely diagenetic effects on oxygen isotopes of chemical transport and mineral-fluid reactions in sediments, applying numerical diffusion-advection-reaction models similar to those developed by Schrag et al. (1995). In more general terms, I have also worked on identifying the controls on the rate of carbonate recrystallization and assessing the extent to which variations in these factors may lead to differential alteration of sediments and geochemical artefacts. I am particularly interested in more accurately modeling the primary chemical heterogeneity of carbonate sediments, as well as the influence of organic rain rate and bottom water carbonate saturation state.

More recently I have also investigated variations in Sr/Ca of bulk carbonates in Cretaceous sediments. Sr/Ca variations in these coccolith-dominated sediments appear to be strongly influenced by productivity variations, although changes in seawater Sr/Ca from rapid sea level falls and reef crises may also contribute. With the development of independent records of past variations in seawater Sr/Ca ratios, carbonate Sr/Ca records may provide a valuable perspective on productivity changes during the Cretaceous.

My work on modeling ocean chemistry has focused on studies of the effect of Quaternary glacial/interglacial sea level variations on geochemical cycles in ocean. This aspect of my doctoral work focused on Sr and Ca budgets of the ocean. An important outcome of this project was prediction of variations in the marine Sr/Ca ratio resulting from sea level changes. The Sr and Ca budgets of the ocean have been assumed to be in steady-state over short timescales; in reality, fractionation of Sr between shelf and deep sea carbonate leads to small but significant deviation from steady state behavior of Sr over glacial sea level cycles. Predicting the magnitude of Sr/Ca variation permits correction of coral Sr/Ca paleotemperature estimates, which have been at odds with other marine and terrestrial climate indicators of glacial climate in the tropics.

Attempts to identify this small variation in seawater Sr/Ca in minor element records of planktonic foraminifera are complicated by non-equilibrium effects on Sr partitioning which require further investigation.

Stoll, H.M. and Ziveri, P. Coccolithophore-based geochemical proxies. in Thierstein, H. and Young, J. Eds.Coccolithophores: From Molecular Processes to Global Impact. Springer Verlag. in review.

Ziveri, P*., Stoll, H.M*., Probert I. , Klaas, C., Geisen, M., Young J., and Ganssen, G.(2003) Stable isotope vital effects in coccolith calcite. Earth and Planetary Science Letters, 210, p. 137-149.

Stoll, H. M., & Schrag, D. P., 1996. Evidence for Glacial Control of Rapid Sea Level Changes in the Early Cretaceous. Science, 272, 1771-1774.

Karabinos, P., Stoll, H.M., and W.T. Fox ,1992. Attracting students to science through field exercises in introductory geology courses. Journal of Geologic Education 40, 302-305.

Stoll, H. and Ziveri, P. 2002. Controls over the Chemistry of Coccolith Calcite. Goldschmidt Conference, Davos, Switzerland, August 2002.

Stoll, H.M., Ziveri, P., Rosenthal, Y., Geisen, M., Probert, I. and Young, J. (2001) "Potential and limitations of Paleoproxies from Sr/Ca Ratios in Coccolith Carbonate." American Geophysical Union Fall Meeting (San Francisco, CA).

Stoll, H.M, Klaas, C., Probert, I., Ziveri, P., Ruiz-Encinar, J., and Garcia-Alonso, J.I. (2001) "Calibration of elemental chemistry (Sr/Ca and Mg/Ca) of coccolith carbonate for paleoclimate studies." European Geophysical Society (EGS) XXVI General Assembly (Nice, France).

Ziveri, P., Probert, I., Klaas, C., Stoll, H., Ganssen, G. and Young, J.(2001) "Species-specific stable isotope composition of coccolith calcite as a paleoceanographic proxy." European Geophysical Society (EGS) XXVI General Assembly (Nice, France).

Ziveri, P., Probert, I., Klaas, C., Stoll, H.M., Ganssen, G.M., and Young, J. (2000). "Species-specific stable isotope composition of coccolith calcite as a paleoceanographic proxy." American Geophysical Union Fall Meeting (San Francisco, CA).

Stoll, H.M, Klaas, C., Ziveri, P., Ruiz-Encinar, J., and Garcia-Alonso, J.I. "Sr/Ca of coccolith carbonate: testing the story of the smallest carbonate repositories." Presentacion oral. International Nannoplankton Association 8th meeting (Bremen, Germany) September 2000.

Ziveri, P., Stoll, H.M., Probert, I., Klaas, C. and Ganssen, G. Is stable isotope composition of coccolith carbonate an effective palaeoceanographic proxy? Presentacion oral. International Nannoplankton Association 8th meeting (Bremen, Alemania) September 2000.

Stoll, H.M, Rosenthal, Y., Klaas, C., Ziveri, P., Baumann, K.H., Boeckel, B., Kinkel, H., and Ganssen, G. "Sr/Ca of coccolith carbonate: testing the story of the smallest carbonate repositories" European Geophysical Society Meeting (Nice, France) April 2000.

Rosenthal, Y., Stoll, H.M., Wyman, K., and Falkowski, P. "Growth Related Variations in Carbon Isotopic Fractionation and Coccolith Chemistry in Emiliania huxleyi", AGU/ASLO 2000 Ocean Sciences Meeting (San Antonio, Texas), January 2000.

Stoll, H.M. and D.P. Schrag. "Coccolith Sr/Ca as a new indicator of coccolithophorid growth and calcification rate", Second Annual CODENET Workshop, Coccolithophorid evolutionary biodiversity and ecology network (Cabara, France) Septembe, 1999.

Stoll, H.M. and D.P. Schrag. "Coccolith Sr/Ca as a new indicator of coccolithophorid growth and calcification rate", Ninth Annual V.W. Goldschmidt Conference on Geochemistry (Boston, USA), August 1999.

Stoll, H.M. and D.P. Schrag. "High resolution stable isotope records from the Upper Cretaceous of Italy and Spain: Glacial episodes in a greenhouse planet", Europen Union of Geosciences Meeting (Strasbourg,France) April 1999.

Stoll, H.M. and D.P. Schrag. "Coccolith Sr/Ca as an Indicator of Marine Paleoproductivity", European Union of Geosciences Meeting, (Strasbourg, France) April 1999.

Stoll, H.M. and D.P. Schrag". Variation in Seawater Sr/Ca over Glacial Cycles: Model Predictions and Foraminiferal Data", American Geophysical Union Fall Meeting, (San Francisco, USADecember 1998.