This report summarizes the use of AAR for the purpose of estimating the ages of macrofossils. Samples for AAR were collected from depths of ~40 to ~80 ft (12.2 to 24.4 m) in the Cape May Zoo corehole. AAR is a qualitative method of estimating ages for samples from the last 1.0 to 1.5 m.y., as reviewed in numerous publications (e.g., Wehmiller and Miller, 2000, and references therein).
Racemization geochronology depends on the phenomenon that amino acids in fossil skeletal material convert from their original 100% "left-handed" configuration to an equal mixture of right- and left-handed amino acids. The rate of this racemization reaction is a function of many variables; therefore, the method is rather qualitative, but with suitable calibration it can be used to estimate ages up to ~1.5 m.y. in the mid-Atlantic region of the United States. A useful example of the use of AAR methods for comparison with the results presented here is the work of O'Neal et al. (2000), who studied the Quaternary stratigraphy of estuarine units along the New Jersey shore of Delaware Bay between the Cohansey and Maurice Rivers.
Samples were obtained from the Cape May Zoo corehole by J.F. Wehmiller. They were prepared and analyzed using standard gas chromatographic methods as reviewed in Wehmiller and Miller (2000). Samples of Mulinia, Ensis, and Mercenaria were obtained from several depths in the core, and after washing and visual examination the best-preserved samples were selected for analysis. The methods employed yield D/L (right/left) values, which increase from 0.0 in living samples to 1.0 in "infinitely old" samples, for up to seven amino acids. The internal consistency of the D/L values for multiple amino acids serves as a qualitative check on the reliability of the results. Samples from ~44.6 ft (13.6 m) were from a sandy and gravelly unit with shell hash. Samples from the greater depths were in a compact mud.
Analytical results (Table T4) show the mean D/L value for each amino acid in each sample analyzed. Data in Table T4 represent modified output from the University of Delaware Aminostratigraphy Lab database. In this table, "Sample" refers to a specific shell, and "Subsample" uniquely identifies any fragment taken from that shell for analysis. In many cases, although not in this study, more than one subsample might be taken from a single shell. Samples are organized by genus and depth in the hole. The amino acids for which values are reported are alloisoleucine (the ratio of D-alloisoleucine to L-isoleucine), alanine, aspartic acid, glutamic acid, leucine, phenylalanine, proline, and valine. "VLPG" is the average of valine, leucine, phenylalanine, and glutamic acid; "Val_Leu" is the average of valine and leucine; "Ala_Asp" is the average of alanine and aspartic acid. In some cases, these averages are a useful way to minimize the "noise" that might arise from the measurements of individual amino acids.
Reported D/L values are mean values from at least two chromatograms for each sample. D/L values are reported by both peak area and peak height. Peak areas are the ratios computed from the measured areas of both the D and L peaks. Peak heights are the ratios computed from the heights of these peaks. Differences between area and height ratios are usually <3%. If the differences are >3%, then an analytical issue (usually an interfering peak on the chromatogram) is identified and the ratios are considered less reliable. Coefficients of variation (CV) are also given in Table T4. The CVs are computed for each genus, combining both area and height values. CVs for Mulinia are usually greater than those for Mercenaria and are usually interpreted to be the result of the fragile nature of Mulinia and the large ratio of surface area to shell mass, which enhances the possibility of open-system diagenesis.
Figure F16 shows the mean D/L values for the Mercenaria and Mulinia results given in Table T4. Because it is known that Mercenaria is a "faster racemizer" than Mulinia (York et al., 1989), we use the relation seen in Figure F16 to conclude that both taxa show internally consistent D/L values (e.g., the "pattern" of D/L among the different amino acids is similar) and that the two groups of D/L values (Mercenaria and Mulinia) represent material of approximately the same geological age. In other words, the spacing of D/L values seen in Figure F16 is consistent with the conclusion that the two groups of samples are equal in age. Ensis is considered to be a "slow racemizer," so the results for the analysis of this single Ensis specimen are consistent with this conclusion.
Because Mercenaria is a much more robust sample than Mulinia, and therefore more geochemically reliable (as often seen by comparing the CV values for the two taxa), we usually focus on Mercenaria results for chronological interpretations. Nevertheless, the Mulinia can be used as a test of the reliability of any conclusions based on Mercenaria.
The robustness of Mercenaria, however, makes it more likely that fragments of this genus will survive reworking and transport (Wehmiller et al., 1995), particularly compared with Mulinia. In the case of the Cape May Zoo corehole results, we use the combination of these characteristics to infer an age for the Mulinia-bearing unit (depth = ~23 m) using not only the results for shells at this depth but also the results for apparently reworked/transported Mercenaria shells at ~44.8 ft (13.66 m) depth. Because the Mercenaria shells are rounded fragments found at the contact between an overlying sandy unit and the underlying mud unit, we conclude that the Mercenaria are found as part of a lag deposit and that they represent the same age as the underlying Mulinia-bearing muddy unit.
The proposed age estimate for the Mercenaria and the "age equivalent" Mulinia is based on a direct comparison with the Mercenaria results presented by O'Neal et al. (2000). Figure F17 shows this comparison by plotting mean D/L values for the Cape May Zoo Mercenaria along with results from the Morie and Unimin Pits (Mauricetown, New Jersey) and two aminozones at Gomez Pit, Virginia that serve as qualitative calibrations. Following the approach of O'Neal et al. (2000), we conclude that the Cape May Zoo shells are roughly equivalent in age to those from Morie and Unimin Pits and that these shells all represent a "pre-last interglacial" age, probably correlative with either MIC 9 or 11 of the marine isotope record, or roughly 300 to 400 ka This conclusion is based on the fact that the D/L values in the New Jersey samples are all greater than those seen in calibration samples (MIC 5; 80 ka) at Gomez Pit (Gomez a in Fig. F17) and quite similar to the D/L values seen in samples from an older unit at Gomez (Gomez c in Fig. F17). Using kinetic models of racemization and the Gomez a calibration point, Gomez c has been estimated to be at least 250 ka in age (Mirecki et al., 1995); correcting for temperature differences between southern New Jersey and southeastern Virginia leads to the conclusion that the New Jersey samples are at least 15% older than those from Gomez c.
Mercenaria and Mulinia were sampled from depths of ~44.6 and 77.1 ft (13.6 and 23.5 m), respectively, in the Cape May Zoo corehole. Although the Mercenaria samples are clearly transported and probably reworked, the combined analysis of all D/L results suggests that Mercenaria and Mulinia can both be used to estimate the age of the Mulinia-bearing unit at ~77.1 ft (23.5 m). Based on comparisons with other AAR data from the United States Atlantic coastal plain, the Mulinia-bearing unit is ~300–400 ka in age. The age of the shallower, sandy unit is not well constrained by the results presented here, as the only samples from this unit (Mercenaria) were taken right at its base and are almost definitely transported or reworked.