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| Titel |
Quantitative Estimates of Temporal Mixing across a 4th-order Depositional Sequence: Variation in Time-averaging along the Holocene Marine Succession of the Po Plain, Italy |
| VerfasserIn |
D. Scarponi, D. Kaufman, J. Bright, M. Kowalewski |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250030952
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| Zusammenfassung |
| Single fossiliferous beds contain biotic remnants that commonly vary in age over a time
span of hundreds to thousands of years. Multiple recent studies suggest that such
temporal mixing is a widespread phenomenon in marine depositional systems. This
research focuses on quantitative estimates of temporal mixing obtained by direct
dating of individual corbulid bivalve shells (Lentidium mediterraneum and Corbula
gibba) from Po plain marine units of the Holocene 4th-order depositional sequence,
including Transgressive Systems Tract [TST] and Highstand Systems Tract [HST].
These units displays a distinctive succession of facies consisting of brackish to
marginal marine retrogradational deposits, (early TST), overlain by fully marine
fine to coarse gray sands (late TST), and capped with progradational deltaic clays
and sands (HST). More than 300 corbulid specimens, representing 19 shell-rich
horizons evenly distributed along the depositional sequence and sampled from 9
cores, have been dated by means of aspartic acid racemization calibrated using 23
AMS-radiocarbon dates (14 dates for Lentidium mediterraneum and 9 dates for Corbula
gibba, respectively).
The results indicate that the scale of time-averaging is comparable when similar depositional
environments from the same systems tract are compared across cores. However, time
averaging is notably different when similar depositional environments from TST and HST
segments of the sequence are compared. Specifically, late HST horizons (n=8) display
relatively low levels of time-averaging: the mean within-horizon range of shell ages is 537
years and standard deviation averages 165 years. In contrast, late TST horizons
(n=7) are dramatically more time-averaged: mean range of 5104 years and mean
standard deviations of 1420 years. Thus, late TST horizons experience a  1 order
of magnitude higher time-averaging than environmentally comparable late HST
horizons.
In conclusion the HST and TST systems tracts of the Po Plain display dramatically different
levels of time-averaging, and therefore, are also likely to differ notably in their
taphonomic overprint. The observed patterns are also consistent with the sequence
stratigraphic paradigm, which predicts differences in rate of sedimentation and
severity of reworking between HST and TST. The results provide a compelling
case for applicability of amino acid racemization methods as a tool for evaluating
changes in depositional dynamics, sedimentation rates, time-averaging, temporal
resolution of the fossil record, and taphonomic overprints across sequence stratigraphic
cycles. |
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