Person:

Jacobsen, Stein

We report initial Sr(87)/Sr(86) values from an Upper Proterozoic carbonate succession from Svalbard and East Greenland. This succession, now tectonically separated into three sequences, is thick, relatively continuous, and well preserved. The relative ages of the samples from within the basin are well constrained by litho-, bio-, and chemostratigraphic techniques. The data from this study and related data from the literature are used to construct a curve of Sr(87)/Sr(86) for Upper Proterozoic seawater. The new data reported in this study substantially improve the isotopic record of Sr in sea water for the period between 650 and 800 Ma. The data indicate that delta Sr(87) values of seawater were variable but low (delta Sr(87) ~ -500 to -250) between 900 and 650 Ma, and rose rapidly to ~ +30 by 600 Ma. The range of variation of delta Sr(*7) in seawater during the Riphean-Vendian exceeds the entire range of delta Sr(87) in seawater during the Phanerozoic. While variation in the average isotopic composition of Sr delivered to the oceans by rivers can account for some of the observed range, changes in the ratio of submarine hydrothermal flux to river water (continental) flux are responsible for the large variation in seawater Sr isotopic composition. Changes in the continental flux of Sr to the oceans can be related to tectonic factors. Large changes in the hydrothermal flux to river water flux ratio indicated by the data could have significant consequences for the chemistry of the ocean-atmosphere system.

New Sr and C isotopic data, both obtained on the same samples of marine carbonates, provide a relatively detailed record of isotopic variation in seawater through the latest Proterozoic and allow, for the first time, direct correlation of these isotopic changes in the Vendian (similar to 540-610 Ma). The strong isotope variations determined in this study record significant environmental and tectonic changes. Together with a fairly poorly constrained Nd isotopic record, the Sr and C isotopic records can be used to constrain rates of erosion, hydrothermal alteration and organic C burial. Further, comparison of these records with those of the Cenozoic permit investigation of the general relationship between global tectonics and continental glaciation. In particular, results of this study show a very large change in the Sr-87/Sr-86 of marine carbonates from low pre-Vendian (> 610 Ma) values (similar to 0.7066) to high Middle Cambrian values (similar to 0.7090). This change is greater in magnitude than the significant increase in seawater Sr-87/Sr-86 through the Cenozoic. Both changes are attributed to high erosion rates associated with continent-continent collisions (Pan-African and Himalayan-Tibetan). In the latest Proterozoic these high erosion rates, probably coupled with high organic productivity and anoxic bottom-water conditions, contributed to a significant increase in the burial rate of organic C. Ice ages mark both the Neoproterozoic and Cenozoic, but different stratigraphic relationships between the Sr isotopic increase and continental glaciation indicate that uplift-driven models proposed to explain Cenozoic climatic change cannot account for the latest Proterozoic ice ages.

Thick, unmetamorphosed successions of siliciclastic and carbonate rocks in eastern and western Siberia preserve a record of Middle Riphean to Early Upper Riphean sedimentary environments and geochemistry. Consistent with data from other continents, our studies in the Uchur-Maya region in southeastern Siberia and the Turukhansk Uplift in northwestern Siberia suggest a first-order shift in delta C-13 from values near 0 parts per thousand in the early Mesoproterozoic to values near +3.5 parts per thousand after about 1300 Ma. Over this same interval, primary Sr-87/Sr-86 values decrease from > 0.7060 to < 0.7053. Combining lithologic, biostratigraphic, and geochemical data sets with available geochronologic constraints, we present a refined correlation between these two key Proterozoic successions in Siberia and add this dataset to a growing body of C and Sr isotopic data from this time interval, Carbon isotope chemostratigraphy from these regions supports the occurrence and timing of a first-order, similar to 3.5 parts per thousand positive shift ca. 1250-1300 Ma, approximately coeval with the onset of active margin activity that predates the main phase of Rodinia assembly. Sr isotopic data may also be interpreted within the context of the evolving Mesoproterozoic tectonic regime. Available data suggest that no dramatic rise in Sr-87/Sr-86 heralds the main phase of Rodinia assembly in the terminal Mesoproterozoic, suggesting that significant juvenile crust was involved in mountain building, that relative hydrothermal flux from mid-ocean ridges remained high throughout the assembly of Rodinia and/or that increased continental runoff related to intense erosion of Grenvillian mountain belts terminated shortly after orogeny.

We report Si-isotopic compositions of 75 sedimentologically and petrographically characterized chert samples with ages ranging from ~2600 to 750 Ma using multi-collector inductively coupled plasma mass spectrometry. (\delta^{30})Si values of the cherts analyzed in this study show a ~7‰ range, from −4.29 to +2.85. This variability can be explained in part by (1) simple mixing of silica derived from continental (higher (\delta^{30})Si) and hydrothermal (lower (\delta^{30})Si) sources, (2) multiple mechanisms of silica precipitation and (3) Rayleigh-type fractionations within pore waters of individual basins. We observe ~3‰ variation in peritidal cherts from a single Neoproterozoic sedimentary basin (Spitsbergen). This variation can be explained by Rayleigh-type fractionation during precipitation from silica-saturated porewaters. In some samples, post-dissolution and reprecipitation of silica could have added to this effect. Our data also indicate that peritidal cherts are enriched in the heavier isotopes of Si whereas basinal cherts associated with banded iron formations (BIF) show lower (\delta^{30})Si. This difference could partly be due to Si being derived from hydrothermal sources in BIFs. We postulate that the difference in (\delta^{30})Si between non-BIF and BIF cherts is consistent with the contrasting genesis of these deposits. Low (\delta^{30})Si in BIF is consistent with laboratory experiments showing that silica adsorbed onto Fe-hydroxide particles preferentially incorporates lighter Si isotopes. Despite large intrabasinal variation and environmental differences, the data show a clear pattern of secular variation. Low (\delta^{30})Si in Archean cherts is consistent with a dominantly hydrothermal source of silica to the oceans at that time. The monotonically increasing (\delta^{30})Si from 3.8 to 1.5 Ga appears to reflect a general increase in continental versus hydrothermal sources of Si in seawater, as well as the preferential removal of lighter Si isotopes during silica precipitation in iron-associated cherts from silica-saturated seawater. The highest (\delta^{30})Si values are observed in 1.5 Ga peritidal cherts; in part, these enriched values could reflect increasing sequestration of light silica during soil-forming processes, thus, delivering relatively heavy dissolved silica to the oceans from continental sources. The causes behind the reversal in trend towards lower (\delta^{30})Si in cherts younger than 1.5 Ga old are less clear. Cherts deposited 1800–1900 Ma are especially low (\delta^{30})Si, a possible indication of transiently strong hydrothermal input at this time.

One explanation of the abrupt cooling episode known as the Younger Dryas (YD) is a cosmic impact or airburst at the YD boundary (YDB) that triggered cooling and resulted in other calamities, including the disappearance of the Clovis culture and the extinction of many large mammal species. We tested the YDB impact hypothesis by analyzing ice samples from the Greenland Ice Sheet Project 2 (GISP2) ice core across the Bølling-Allerød/YD boundary for major and trace elements. We found a large Pt anomaly at the YDB, not accompanied by a prominent Ir anomaly, with the Pt/Ir ratios at the Pt peak exceeding those in known terrestrial and extraterrestrial materials. Whereas the highly fractionated Pt/Ir ratio rules out mantle or chondritic sources of the Pt anomaly, it does not allow positive identification of the source. Circumstantial evidence such as very high, superchondritic Pt/Al ratios associated with the Pt anomaly and its timing, different from other major events recorded on the GISP2 ice core such as well-understood sulfate spikes caused by volcanic activity and the ammonium and nitrate spike due to the biomass destruction, hints for an extraterrestrial source of Pt. Such a source could have been a highly differentiated object like an Ir-poor iron meteorite that is unlikely to result in an airburst or trigger wide wildfires proposed by the YDB impact hypothesis.