Browsing by Author "Whitacre, KE"

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    One fossil record, multiple time resolutions: disparate time-averaging of echinoids and mollusks on a Holocene carbonate platform
    (Geological Society of America, 2017-11-11) Kowalewski, M; Casebolt, S; Hua, Q; Whitacre, KE; Kaufman, DS; Kosnik, MA
    Our quantitative understanding of the temporal resolution of the fossil record is largely based on numerical dating of Holocene bivalves. However, for paleontologically important taxa with less-robust skeletons, no quantitative estimate of time-averaging exists. Here, we compare time-averaging in sympatric accumulations of the echinoid Leodia sexiesperforata and the bivalve Tucetona pectinata on a shallow carbonate platform of San Salvador Island, Bahamas. Using graphite-target and carbonate-target accelerator mass spectrometry (AMS) radiocarbon methods, we dated 30 echinoid tests and 30 mollusk valves collected from surficial sediments at a single site. All echinoid tests yielded post-bomb (taking into account radiocarbon from nuclear weapons testing) ages, and the estimated time-averaging (interquartile age range) is at most 2 yr. In contrast, sympatric mollusk valves spanned 4000 yr with an estimated time-averaging of 1830 yr. This three-orders-of-magnitude difference in the extent of time-averaging quantifies the taphonomic expectation that echinoid tests should degrade more rapidly and experience less time-averaging when compared to more durable mollusk shells. When preserved, fossil echinoids are likely to indicate a more finely resolved fossil record than skeletally robust organisms such as mollusks, and may provide a fundamentally distinct class of paleontological data, potentially adequate for investigating biological and physical processes that operate at subdecadal time scales. Immensely disparate time-averaging of sympatric mollusks and echinoids indicates that, at broader phylogenetic scales, differences in intrinsic skeletal durability may be the main determinant of the temporal resolution of fossil assemblages. Copyright © 2017 Geological Society of America
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    Radiocarbon-calibrated amino acid racemization ages from Holocene sand dollars (Peronella peronii)
    (Elsevier, 2017-04) Kosnik, MA; Hua, Q; Kaufman, DS; Kowalewski, M; Whitacre, KE
    Amino acid racemization (AAR) is widely used as a cost-effective method to date molluscs in time-averaging and taphonomic studies, but it has not been attempted for echinoderms despite their paleobiological importance and distinct biomineralization. Here we demonstrate the applicability of AAR geochronology for dating Holocene Peronella peronii (Echinodermata: Echinoidea) collected from Sydney Harbour (Australia). Using standard HPLC methods we determined the extent of AAR in 74 Peronella tests and performed replicate analyses on 23 tests. Replicate analyses from the outer edge of 23 tests spanning the observed range of D/L values yielded median coefficients of variation <4% for Asp, Glu, and Phe D/L values, which is comparable to the analytical precision. Correlations between THAA D/L values across 178 independently prepared subsamples of 74 individuals are also very high (Spearman ρ ≥ 0.95) for these three amino acids. The ages of 20 individuals spanning the observed range of D/L values were determined using 14C analyses, and Bayesian model averaging was used to determine the best AAR age model. Only three models fit to Glu D/L contributed to the final averaged age model. Modeled ages ranged from 14 to 5496 years, and the median 95% confidence interval for the 54 AAR ages was ±29% of the modelled age. In comparison, the median 2σ confidence interval for nine graphite target 14C ages was ±8% of the median age estimate and the median 2σ confidence interval for 20 carbonate target 14C ages was ±26% of the median age estimate. Overall Peronella yield high-quality D/L values and appear to be a good target for AAR geochronology. ©2016 Elsevier B.V
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    Time-averaging and stratigraphic resolution in death assemblages and Holocene deposits: Sydney Harbour’s molluscan record
    (GeoScience World, 2016-11-01) Dominguez, JG; Kosnik, MA; Allen, AP; Hua, Q; Jacob, DE; Kaufman, DS; Whitacre, KE
    Time-averaging has evolved from an unrecognized variable in paleoecological analyses to a key concept in understanding the dynamics of sedimentary systems and the formation of fossil deposits. Here we used radiocarbon-calibrated amino acid racemization ages from 173 Fulvia tenuicostata shells collected from Sydney Harbour (NSW, Australia) to quantify time-averaging in surficial and excavated death assemblages. A novel approach to estimating the total age-estimate variability of a collection of specimens is presented that integrates the age-estimation error associated with the AAR calibration model and the effect of time-averaging variability on the age distribution. Fulvia collected from a single 1.6-m deep excavation were used to quantify changes in time-averaging with burial depth. Fulvia collected from surficial death assemblages at six sites were used to quantify spatial variation in time-averaging. The median shell age increased from ∼ 150 yr to ∼ 4230 yr and time-averaging from ∼ 40 yr to ∼ 960 yr with increased burial depth. While four sites contained surface shell assemblages with median ages of ∼ 150 yr and time-averaging of ∼ 40 yr, two sites had death assemblages with older median ages and time-averaging > 1900 yr. A decline in the abundance of Fulvia in the post-colonial period is likely responsible for the rarity of very young shells in the surface death assemblages, while local factors such as dredging and other human activities are likely responsible for the total age-estimate variability in the two highly time-averaged surface collections. These analyses provide a geochronological context for assessing ecological changes in the harbor's benthic communities since colonization and offer an important perspective on the formation of Holocene fossil deposits. © 2016 GeoScience World