Formation of a Slushball: the Process and the Influence of Continental Configuration
Most studies that analyze the stability of 'slushball' Earth have been done with a climate model that did not include an active ice sheet module. These models can thus only generate ice on the low latitude continents spontaneously, neglecting the fact that ice can also be generated by ice flow from large ice sheets at higher latitude. By incorporating ice sheet dynamics in the model, we find that low latitude land ice can form at a significantly higher CO2 concentration than if it is not included, therefore, greatly increases the possibility of the formation of a slushball. Since the ice sheet dynamics is affected by continental configuration, we find that two conditions are most critical: (1) the individual continental fragments must be sufficiently connected so as to allow the expansion of land ice from high latitude to low latitude. (2) There is appropriate fraction of continental mass distributed in the mid- to high latitude so that ice sheets can be initiated at a relatively higher CO2 concentration. As a case study, slushball solutions for a realistic 720 Ma continental configuration appropriate for the Sturtian glaciation exist when the super-continent is not centered exactly on the equator but rather shifted from it by more than 10 degree but less than 20 degree and the connectivity of the individual continental fragments is sufficient.
PP73A-02 [Moved to PP71A]
Origin of the Squantum 'Tillite', Massachusetts, USA: Modern Analogs and Implications for Neoproterozoic Climate Models
A central challenge to the 'Snowball Earth' hypothesis is whether the sedimentary rocks deposited during the Neoproterozoic (c. 750-570 Ma) are glacial tillites that accumulated under global ice sheets during this era. This uncertainty stems from the fact that diamictites are not uniquely glacial in origin, as the slumping and mixing of sediment downslope can also produce diamictites. A key deposit in this debate is the Squantum 'tillite' (ca. 595-570 Ma) preserved in the Boston Basin in Massachusetts, USA, which originated as an arc- related basin within the Avalon island arc terrane during the Neoproterozoic. Detailed field examinations of the Squantum by the author suggest that it owes its origin to the downslope transport of large volumes of unstable volcanic and sedimentary debris from steep basin margin slopes. No evidence of a glacial environment was identified. Thin-section analysis of this deposit has revealed a significant volcanic influence on sedimentation in the form of hitherto unrecognized volcanic lapilli tuff horizons and turbidites consisting of reworked ash in strata associated with Squantum diamictite. These results point to deposition related to tectonic activity and basin development rather than severe global glacial conditions. In light of these results, the Squantum diamictite was compared to the volcaniclastic mass flows deposits exposed along the active Lesser Antilles Arc in the Caribbean. Many of these flows are transported into the adjacent Grenada back-arc Basin by debris flows and turbidity currents resulting in the deposition of volcaniclastic conglomerates, diamictites and thin ash turbidites. Gross stratigraphic and sedimentological similarities of the mass flow facies in the Caribbean can be identified with the Squantum deposits, suggesting that appropriate depositional analogs for the Squantum can be found along the Lesser Antilles Arc. The significance of these results is that they emphasize the importance of detailed field examination of deposits uncritically labeled as Neoproterozoic 'tillites' by paleoclimate modelers.
Aptian Carbon and Sulfur Isotope Curves From ODP Site 765
δ13C and δ34S ratios measured on samples from various ODP Sites suggest that major
shifts in the partitioning between the reduced and oxidized reservoirs of carbon and sulfur occurred during the
Early Cretaceous (Paytan et al. 2004, Wortmann & Chernyavsky 2007). However, the Cretaceous
δ13C and δ34S signals are derived from datasets from different localities. The lack of a
comprehensive dataset and the vastly different residence times of C and S make identifying coupling of the C
and S cycle difficult. Here we construct a high resolution record of the δ13C and δ34S
signals, where both measurements are obtained from the same samples.
312 samples from ODP Site 765 were milled and divided in half. One half was decarbonated, and
measurements of δ13C were performed on the remaining bulk organic carbon. Our δ13C
data show the typical Aptian carbon isotopic signature, a negative spike followed by a positive peak. A marine
barite separation following the procedure of Paytan (1996) was performed on the other half of a subset of 48 of
the samples from before and after the negative excursion. The samples were assessed for evidence of
diagenesis using an SEM/EDS and δ34S was measured on the 24 barite samples of nondiagenetic
Our data include new data points that have a more negative isotopic signature than predicted by the Paytan et
al. (2004) data, indicating complexity in the δ34S curve not previously observed. The data also
support the notion that the δ13C and δ34S excursions are causally related. Future work
refining these measurements will continue to help constrain the temporal relationship between the C and S
cycles on timescales of ∼1 million years.
Paytan, A. (1996). Marine barite: a recorder of oceanic chemistry, productivity and circulation. Ph.D. Thesis,
Univ. of Calif., San Diego, California.
Paytan, A., M. Kastner, et al. (2004). Seawater sulfur isotope fluctuations in the Cretaceous. Science 304(5677): 1663-1665.
Wortmann, U. G. and B. M. Chernyavsky (2007). Effect of evaporite deposition on Early Cretaceous carbon and sulphur cycling. Nature 446(7136): 654-656.
Seawater Exchange and Freshwater Input to the Eocene Arctic Ocean from Nd-Sr Isotope Proxies in Fossil Fish Debris
The neodymium isotopic composition of Eocene Arctic Intermediate Water (AIW) has been determined by proxy compositions measured in early to middle Eocene ichthyoliths from Lomonosov Ridge (IODP Arctic Coring Expedition 302). εNd compositions are distinctly more radiogenic than modern AIW, suggesting several possible sources of Nd to the Eocene Arctic Ocean. Periodic marine inputs from Tethys, the Pacific, and North Atlantic (or Norwegian-Greenland Sea) all likely contributed to the Eocene AIW Nd signature at Lomonosov Ridge between 55 and 45 Ma, in contrast to the prevailing view that exchange was highly restricted. The Eocene Sr isotopic record at Lomonosov Ridge is, in contrast to the Nd record, consistent with fresh to brackish surface water conditions persisting for ca. 10 million years at this location. These data suggest strong decoupling in the sources of Nd and Sr in fish debris, consistent with a salinity-stratified (and strontium- stratified) water column above Lomonosov Ridge. 87Sr/86Sr values of ichthyoliths are more radiogenic than Eocene seawater and probably record the overall balance of river Sr flux to the surface Eocene Arctic Ocean. These data support previous conclusions that fresh water inputs via direct precipitation and rivers combined with unprecedented warmth to support a dynamic, largely ice-free Arctic hydrologic cycle during the early and middle Eocene.
PP73A-05 [Moved to PP71B]
Mg/Ca Ratios in Coralline Red Algae as Temperature Proxies for Reconstructing Labrador Current Variability
Marine ecosystems and fishery productivity in the Northwestern Atlantic have been considerably affected by regional climate and oceanographic changes. Fluctuations of North Atlantic marine climate have been linked in part to a dominant pattern of atmospheric circulation known as the North Atlantic Oscillation, which has a strong influence on transport variability of the Labrador Current (LC). The cold LC originates in the Labrador Sea and flows southbound along the Eastern Canadian coastline causing an important cooling effect on marine waters off the Canadian Atlantic provinces. Although interdecadal and interannual variability of sea surface temperatures (SST) in the LC system have been documented, a long-term pattern has not been identified. In order to better understand the observed ecosystem changes and their relationship with climate variability in the Northwestern Atlantic, a century-scale reconstruction of spatial and temporal variations of the LC is needed. This, however, requires reliable long-term and high-resolution SST records, which are not available from short instrumental observations. Here we present the first century-scale SST reconstructions from the Northwest Atlantic using long-lived coralline red algae. Coralline red algae have a high-Mg calcite skeleton, live in shallow water worldwide and develop annual growth bands. It has previously been demonstrated that subannual resolution SSTs can be obtained from coralline red algal Mg/Ca ratios, a commonly used paleotemperature proxy. Specimens of the long-lived coralline red algae Clathromorphum compactum were collected alive in August 2008 along a latitudinal transect spanning the southern extent of LC flow in Nova Scotia and Newfoundland. This collection is supplemented with specimens from the same region collected in the 1960's. In order to reconstruct spatial and temporal patterns of the LC, selected samples of C. compactum were analyzed for Mg/Ca using Laser Ablation Inductively-Coupled Plasma Mass Spectrometry (LA-ICP-MS). Mg/Ca ratios range from 0.048 to 0.138 (measured in weight %) and relate to water temperatures of -1 to 16°C. Age models were established by comparing annual growth increments (average increment width 350 microns/year) with Mg/Ca cycles. This yielded subannually-resolved Mg/Ca-based SST reconstructions spanning the past century.
Spectral Analysis of Burgundy Pinot Noir Grape Harvest Dates
We perform an analysis of Burgundy pinot noir grape harvest dates. This annual record reports the annual grape harvest date in days after September 1st, which has been used in summer temperature reconstructions. We perform spectral analysis and dynamic spectral analysis of the data where we find evidence of coherence indicating solar influence. Specifically we find structure in the dynamic spectra indicating solar influence, and there is evidence of the non-stationary structure associated with Seuss cycles as seen in other historical temperature data.