Quantifying Time-Averaging In 4th-Order Depositional Sequences: Radiocarbon-Calibrated Amino-Acid Racemization Dating of Late Quaternary Mollusk Shells from Po Plain, Italy

Quantitative estimates of time-averaging (age mixing) in the Holocene marine successions of Po Plain were obtained by direct dating of individual specimens of the corbulid bivalve Lentidium mediterraneum sampled across the 4th-order depositional sequence. The sequence displays a distinct succession of facies consisting of brackish to marginal marine retrogradational deposits (early Transgressive Systems Tract [early-TST]), overlain by fully marine sands (late transgressive systems tract [late-TST]), and capped with progradational deltaic clays and sands (highstand systems tract [HST]). More than 230 specimens of L. mediterraneum, representing 22 shell-rich horizons evenly distributed along the depositional sequence and sampled from 9 cores, have been dated using amino acid (aspartic acid) racemization ratios calibrated with 14 AMS-radiocarbon dates.

The results indicate that the scale of time-averaging is notably different when similar depositional environments from TST (including both early-TST and late-TST horizons) and HST segments of the sequence are compared. HST horizons (n=12) display relatively low levels of time-averaging: the mean within-horizon range of shell ages is 477 years and standard deviation averages 163 years. In contrast, TST horizons (n=9) are dramatically more time-averaged: mean range of 5067 and mean standard deviations of 1380 years. Thus, time-averaging in TST horizons is about an order of magnitude higher relative to environmentally comparable HST horizons.

The HST and TST systems tracts of 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.