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Seminar Speaker: Dr. Nathan Sheldon
21 April, 2017 @ 3:30 pm - 4:30 pm
University of Michigan
Earth and Environmental Sciences
Seminar Talk: “When did the terrestrial biosphere become important to global biogeochemistry?”
In the modern oceans, either phosphorus (P) or nitrogen (N) may act as a limiting nutrient. While most N is generated in situ by N fixing bacteria, the supply of P is controlled almost entirely by the flux of P from the continents. Thus, the intensity of continental weathering processes exerts a large-scale control on global biogeochemistry and on the rates of both carbon burial and oxygen production. Two primary models have been proposed for when the terrestrial biosphere changed the P economics of the ocean, either early in the Archean (i.e., origin of terrestrial life) or with the advent of land plants in Silurian. It is possible to test whether either of these models is correct using a combination of paleosol (fossil soil), paleo-lake, and modern analogue systems, and to compare them directly to evidence for changing P flux to the oceans. While Archean paleosols show evidence of biological activity and of significant localized elemental cycling, the limited depth of the biosphere (i.e., mm to cm) and the limited extent of habitable surface environments means that no significant P flux is observed. However, by 1.8 Ga ago (Paleoproterozoic) paleosol P cycling dynamics are remarkably similar to modern soils. In contrast, lake environments throughout the Paleoproterozoic continue to exhibit limited biological productivity and frequent anoxic or euxinic conditions that inhibited productivity, with some higher productivity paleo-lakes recognized by the late Mesoproterozoic (1.2-1.1 Ga ago). Marine ecosystems in contrast to both paleosol and paleo-lake records, show no increase in productivity until the Neoproterozoic, when both carbon isotope and P abundance data undergo a state shift toward modern values. Taken together, these various results suggest that a terrestrial biosphere capable of delivering significant P to the oceans was in place well before it actually happened. Possible explanations for this lag include insufficient shallow marine shelf areas for significant productivity until the Neoproterozoic (plenty of P, not enough marine primary producers) or insufficient habitable surface area in terrestrial environments (high local P flux, low global P flux). A final intriguing possibility that is supported by modern analogue systems is that the environmental stoichiometry of the ancient ecosystems was fundamentally different and that carbon and iron may have been limiting nutrients in many systems in addition to P.