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Milankovitch Modulation of the Ecosystem Dynamics of Fossil Great Lakes

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Abstract

Triassic and Early Jurassic lacustrine deposits of eastern North American rift basins preserve a spectacular record of precession-related Milankovitch forcing in the Pangean tropics. The abundant and well-preserved fossil fish assemblages from these great lakes demonstrate a sequence of cyclical changes that track the permeating hierarchy of climatic cycles. To detail ecosystem processes correlating with succession of fish communities, we measured bulk delta13Corg through a 100 ky series of Early Jurassic climatic precession-forced lake level cycles in the lower Shuttle Meadow Formation of the Hartford rift basin, CT. The deep-water phase of one of these cycles, the Bluff Head bed, has produced thousands of articulated fish. We observe fluctuations in the bulk delta13Corg of the cyclical strata that reflect differing degrees of lake water stratification, nutrient levels, and relative proportion of algal vs. plant derived organic matter that trace fish community changes. We can exclude extrinsic changes in the global exchangeable reservoirs as an origin of this variability because molecule-level delta13C of n-alkanes of plant leaf waxes from the same strata show no such variability. While at higher taxonomic levels the fish communities responded largely by sorting of taxa by environmental forcing, at the species level the holostean genus Semionotus responded by in situ evolution, and ultimately extinction, of a species flock. Fluctuations at the higher frequency, climatic precessional scale are mirrored at lower frequency, eccentricity modulated, scales, all following the lake-level hierarchical pattern. Thus, lacustrine isotopic ratios amplify the Milankovitch climate signal that was already intensified by sequelae of the end-Triassic extinctions. The degree to which the ecological structure of modern lakes responds to similar environmental cyclicity is largely unknown, but we suspect similar patterns and processes within the Neogene history of the East African great lakes, which may be modified in the future by anthropogenic CO2-driven intensification of the hydrological cycle.
Milankovitch Modulation of the Ecosystem Dynamics of Fossil
Great Lakes
Details
Meeting 2008 Fall Meeting
Section
Session
Paleoceanography and Paleoclimatology
Mesozoic/Early Cenozoic Geochemical Records of Paleoclimatic and Paleoceanographic
Variability II Posters
Identifier
PP33B-1535
Authors
Whiteside, J H*, Brown University, Department of Geological Sciences, 324 Brook Street,
Box 1846, Providence, RI 02912, United States
Olsen, P E, Lamont-Doherty Earth Observatory of Columbia University, Department of
Earth and Environmental Sciences, 61 Route 9W, Palisades, NY 10964, United States
Eglinton, T I, Woods Hole Oceanographic Institution, Department of Marine Chemistry
and Geochemistry, Fye 111, MS #4, Woods Hole, MA 02543, United States
Cornet, B, El Paso Community College, Department of Geology, 16921 Cielito Lindo, El
Paso, TX 79938, United States
Huber, P, Geoscience Books, PO Box 1036, Fairbault, MN 55021, United States
McDonald, N G, Westminster School, 995 Hopmeadow Street, Simsbury, CT 06070,
United States
Abstract
Triassic and Early Jurassic lacustrine deposits of eastern North American rift basins preserve a
spectacular record of precession-related Milankovitch forcing in the Pangean tropics. The abundant and
well-preserved fossil fish assemblages from these great lakes demonstrate a sequence of cyclical changes
that track the permeating hierarchy of climatic cycles. To detail ecosystem processes correlating with
succession of fish communities, we measured bulk δ
13
C
org
through a 100 ky series of Early Jurassic
climatic precession-forced lake level cycles in the lower Shuttle Meadow Formation of the Hartford rift
basin, CT. The deep-water phase of one of these cycles, the Bluff Head bed, has produced thousands of
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articulated fish. We observe fluctuations in the bulk δ
13
C
org
of the cyclical strata that reflect differing
degrees of lake water stratification, nutrient levels, and relative proportion of algal vs. plant derived
organic matter that trace fish community changes. We can exclude extrinsic changes in the global
exchangeable reservoirs as an origin of this variability because molecule-level δ
13
C of n-alkanes of
plant leaf waxes from the same strata show no such variability. While at higher taxonomic levels the fish
communities responded largely by sorting of taxa by environmental forcing, at the species level the
holostean genus Semionotus responded by in situ evolution, and ultimately extinction, of a species flock.
Fluctuations at the higher frequency, climatic precessional scale are mirrored at lower frequency,
eccentricity modulated, scales, all following the lake-level hierarchical pattern. Thus, lacustrine isotopic
ratios amplify the Milankovitch climate signal that was already intensified by sequelae of the
end-Triassic extinctions. The degree to which the ecological structure of modern lakes responds to
similar environmental cyclicity is largely unknown, but we suspect similar patterns and processes within
the Neogene history of the East African great lakes, which may be modified in the future by
anthropogenic CO
2
-driven intensification of the hydrological cycle.
Cite as: Author(s) (2008), Title, Eos Trans. AGU, 89(53), Fall Meet. Suppl., Abstract PP33B-1535
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