Jeffrey S. Pigati

Radiocarbon dating of minute gastropods and new constraints on the timing of late Quaternary spring-discharge deposits in southern Arizona, USA

Abstract Gastropod shells are commonly preserved in Quaternary sediments, but are often avoided for radiocarbon dating because some taxa incorporate 14 C-deficient carbon during shell formation. Recently, Brennan and Quade [(1997) Quat. Res. 47, 329–336] found that some minute taxa ( Vallonia , Pupilla , and Succineidae ) appear to yield reliable 14 C ages for late Pleistocene samples. A more rigorous evaluation of the 14 C inventory of minute gastropods is presented here, which involved measuring the 14 C activity of specimens collected live in two geologic settings that maximize the potential for ingestion of ‘old’ carbon: (1) alluvium dominated by Paleozoic carbonate rocks, and (2) adjacent to extant springs with highly 14 C-deficient water present at the surface. We found that several minute taxa, including Vallonia , incorporate significant and variable amounts of old carbon (∼2 to >30%) during shell formation. The 14 C activities of the land snails Pupilla blandi and Euconulus fulvus , however, are indistinguishable from the 14 C activity of live plants. The 14 C activity of the semi-aquatic gastropod Catinella sp. (Family: Succineidae ) deviates from modern values in the presence of 14 C-deficient water by an amount equivalent to ∼10% of the local carbon-reservoir effect. These results imply that at least some minute gastropods can provide reliable 14 C ages even when 14 C-deficient carbon is readily available. To demonstrate an application of our findings, we 14 C-dated shells from P. muscorum , E. fulvus , and Succinea sp. (Family: Succineidae ) recovered from the Coro Marl, a late Pleistocene spring-fed marsh deposit exposed at the Murray Springs Paleoindian site in the San Pedro Valley of southern Arizona, USA. Radiocarbon ages obtained from the minute gastropods show that the unit was deposited between ∼25 000 and 13 000 14 C years ago. The marl is situated >15 m above the modern water table at Murray Springs, and is similarly positioned in discontinuous outcrops along a ∼150-km stretch of the San Pedro Valley. Thus, the 14 C ages of minute gastropods presented here may be used to infer the timing of high water-table levels throughout the valley.

Re-evaluation of mid-Holocene deposits at Quebrada Puripica, northern Chile

Duringthe middle Holocene (8^3 ka), wetland deposits accumulated in areas with emerg ent water tables in the central Atacama Desert (22^24‡S), producinga stratig raphic unit (Unit C) that can be mapped and correlated across different basins and geomorphic settings. Wherever mapped, Unit C is located between 6 and 30 m above modern wetlands, and includes thick sequences of diatomite and organic mats. The origin, depositional environment, and paleoclimatic significance of Unit C is controversial and currently under debate. Grosjean [Science 292 (2001) 2391a] suggests the unit developed under a regime of falling lake and ground-water levels, whereas evidence presented here suggests that Unit C formed during a period of rising ground-water levels and increased vegetation cover. The debate is embedded in broader discussions about geomorphic processes in ground-water-fed streams, as well as the history and forcingof climate variability in the central Andes. The central Atacama and Andes are remote, with few opportunities for different researchers to examine the same site. One exception is a sequence of mid-Holocene deposits at the confluence of Quebrada Seca and R| ¤o Puripica in northern Chile (23‡S). Grosjean et al. [Quat. Res. 48 (1997) 239^246] initially suggested the deposits accumulated in temporary lake basins, which formed when recurringdebris flows from a side canyon (Quebrada Seca) dammed the main channel (R|¤o Puripica) duringa period of drought and reduced stream flow. Upon our visit to the site, we found evidence that clearly demonstrates the deposits were not formed in temporary lakes, but rather were deposited in a wetland environment. Disagreement remains about the climatic interpretation of wetland deposits. Grosjean [Science 292 (2001) 2391a] now suggests that local ground-water levels rise and wetland deposits aggrade in deep canyon systems, such as R| ¤o Puripica, when stream power and channel erosion is reduced duringprolong ed dry spells. However, sedimentological evidence and the presence of Unit C in many depositional environments, not just deep canyons, indicate that it formed during a period of higher regional ground-water levels that were sustained by enhanced precipitation and recharge in the High Andes. ? 2003 Elsevier Science B.V. All rights reserved.

Accumulation of impact markers in desert wetlands and implications for the Younger Dryas impact hypothesis

The Younger Dryas impact hypothesis contends that an extraterrestrial object exploded over North America at 12.9 ka, initiating the Younger Dryas cold event, the extinction of many North American megafauna, and the demise of the Clovis archeological culture. Although the exact nature and location of the proposed impact or explosion remain unclear, alleged evidence for the fallout comes from multiple sites across North America and a site in Belgium. At 6 of the 10 original sites (excluding the Carolina Bays), elevated concentrations of various “impact markers” were found in association with black mats that date to the onset of the Younger Dryas. Black mats are common features in paleowetland deposits and typically represent shallow marsh environments. In this study, we investigated black mats ranging in age from approximately 6 to more than 40 ka in the southwestern United States and the Atacama Desert of northern Chile. At 10 of 13 sites, we found elevated concentrations of iridium in bulk and magnetic sediments, magnetic spherules, and/or titanomagnetite grains within or at the base of black mats, regardless of their age or location, suggesting that elevated concentrations of these markers arise from processes common to wetland systems, and not a catastrophic extraterrestrial impact event.

Chronology, sedimentology, and microfauna of groundwater discharge deposits in the central Mojave Desert, Valley Wells, California

During the late Pleistocene, emergent groundwater supported persistent and long-lived desert wetlands in many broad valleys and basins in the American Southwest. When active, these systems provided important food and water sources for local fauna, supported hydrophilic and phreatophytic vegetation, and acted as catchments for eolian and alluvial sediments. Desert wetlands are represented in the geologic record by groundwater discharge deposits, which are also called spring or wetland deposits. Groundwater discharge deposits contain information on the timing and magnitude of past changes in water-table levels and, thus, are a source of paleohydrologic and paleoclimatic information. Here, we present the results of an investigation of extensive groundwater discharge deposits in the central Mojave Desert at Valley Wells, California. We used geologic mapping and stratigraphic relations to identify two distinct wetland sequences at Valley Wells, which we dated using radiocarbon, luminescence, and uranium-series techniques. We also analyzed the sediments and microfauna (ostracodes and gastropods) to reconstruct the specifi c environments in which they formed. Our results suggest that the earliest episode of high water-table conditions at Valley Wells began ca. 60 ka (thousands of calendar yr B.P.), and culminated in peak discharge between ca. 40 and 35 ka. During this time, cold (4‐12 °C) emergent groundwater supported extensive wetlands that likely were composed of a wet, sedge-rush-tussock meadow mixed with mesic riparian forest. After ca. 35 ka, the water table dropped below the ground surface but was still shallow enough to support dense stands of phreatophytes through the Last Glacial Maximum (LGM). The water table dropped further after the LGM, and xeric conditions prevailed until modest wetlands returned briefl y during the Younger Dryas cold event (13.0‐11.6 ka). We did not observe any evidence of wet conditions during the Holocene at Valley Wells. The timing of these fl uctuations is consistent with changes in other paleowetland systems in the Mojave Desert, the nearby Great Basin Desert, and in southeastern Arizona, near the border of the Sonoran and Chihuahuan Deserts. The similarities in hydrologic conditions between these disparate locations suggest that changes in groundwater levels during the late Pleistocene in desert wetlands scattered throughout the American Southwest were likely driven by synopticscale climate processes.

A Simplified In Situ Cosmogenic 14C Extraction System

We describe the design, construction, and testing of a new, simplified in situ radiocarbon extraction system at the University of Arizona. Blank levels for the new system are low ((234 ± 11) x 10^3 atoms (1 σ; n = 7)) and stable. The precision of a given measurement depends on the concentration of 14C, but is typically <5% for concentrations of 100 x 10^3 atoms g^(-1) or more. The new system is relatively small and easy to construct, costs significantly less than the original in situ 14C extraction system at Arizona, and lends itself to future automation.

Assessing open-system behavior of 14C in terrestrial gastropod shells

In order to assess open-system behavior of radiocarbon in fossil gastropod shells, we measured the 14C activity on 10 aliquots of shell material recovered from Illinoian (~190-130 ka) and pre-Illinoian (~800 ka) loess and lacustrine deposits in the Midwestern USA. Eight of the 10 aliquots yielded measurable 14C activities that ranged from 0.25 to 0.53 per- cent modern carbon (pMC), corresponding to apparent 14C ages between 48.2 and 42.1 ka. This small level of open-system behavior is common in many materials that are used for 14C dating (e.g. charcoal), and typically sets the upper practical limit of the technique. Two aliquots of gastropod shells from the Illinoian-aged Petersburg Silt (Petersburg Section) in central Illi- nois, USA, however, yielded elevated 14C activities of 1.26 and 1.71 pMC, which correspond to apparent 14C ages of 35.1 and 32.7 ka. Together, these results suggest that while many fossil gastropods shells may not suffer from major (>1%) open-sys- tem problems, this is not always the case. We then examined the mineralogy, trace element chemistry, and physical charac- teristics of a suite of fossil and modern gastropod shells to identify the source of contamination in the Petersburg shells and assess the effectiveness of these screening techniques at identifying samples suitable for 14C dating. Mineralogical (XRD) and trace element analyses were inconclusive, which suggests that these techniques are not suitable for assessing open-system behavior in terrestrial gastropod shells. Analysis with scanning electron microscopy (SEM), however, identified secondary mineralization (calcium carbonate) primarily within the inner whorls of the Petersburg shells. This indicates that SEM exam- ination, or possibly standard microscope examination, of the interior of gastropod shells should be used when selecting fossil gastropod shells for 14C dating.

On correcting 14C ages of gastropod shell carbonate for fractionation

Correcting the (super 14) C age of a sample for fractionation is straightforward if the measured carbon was derived entirely from the atmosphere, either directly or through chemical and/or biological reactions that originated with atmospheric carbon. This correction is complicated in the case of gastropods that incorporate carbon from limestone or secondary carbonate (e.g. soil carbonate) during shell formation. The carbon isotopic composition of such gastropod shells is determined by fractionation, as well as mixing of carbon from sources with different isotopic values. Only the component of shell carbonate derived from atmospheric carbon should be corrected for fractionation. In this paper, the author derives a new expression for correcting the measured (super 14) C activity of gastropod shells for fractionation, and describe an iterative approach that allows the corrected (super 14) C activity and the fraction of shell carbonate derived from atmospheric carbon to be determined simultaneously.

Dynamic response of desert wetlands to abrupt climate change

Significance A paleohydrologic record from the Las Vegas Valley (southern Nevada, United States) shows that desert wetlands were extremely sensitive to rapid climatic changes during the past 35,000 years and exhibit temporal congruence with the Greenland ice core record. Wetlands in the valley expanded and contracted many times in response to climatic events during this period, including Dansgaard–Oeschger cycles and other millennial and submillennial climatic perturbations. Widespread erosion occurred when entire wetland systems collapsed during exceptionally dry times. Drought-like conditions typically lasted for a few centuries, which highlights the threat of anthropogenic warming to endemic fauna and flora that depend on desert wetlands to provide a consistent source of water in an otherwise arid landscape. Desert wetlands are keystone ecosystems in arid environments and are preserved in the geologic record as groundwater discharge (GWD) deposits. GWD deposits are inherently discontinuous and stratigraphically complex, which has limited our understanding of how desert wetlands responded to past episodes of rapid climate change. Previous studies have shown that wetlands responded to climate change on glacial to interglacial timescales, but their sensitivity to short-lived climate perturbations is largely unknown. Here, we show that GWD deposits in the Las Vegas Valley (southern Nevada, United States) provide a detailed and nearly complete record of dynamic hydrologic changes during the past 35 ka (thousands of calibrated 14C years before present), including cycles of wetland expansion and contraction that correlate tightly with climatic oscillations recorded in the Greenland ice cores. Cessation of discharge associated with rapid warming events resulted in the collapse of entire wetland systems in the Las Vegas Valley at multiple times during the late Quaternary. On average, drought-like conditions, as recorded by widespread erosion and the formation of desert soils, lasted for a few centuries. This record illustrates the vulnerability of desert wetland flora and fauna to abrupt climate change. It also shows that GWD deposits can be used to reconstruct paleohydrologic conditions at millennial to submillennial timescales and informs conservation efforts aimed at protecting these fragile ecosystems in the face of anthropogenic warming.

Evidence of Repeated Wildfires Prior to Human Occupation on San Nicolas Island, California

Abstract. Understanding how early humans on the California Channel Islands might have changed local fire regimes requires a baseline knowledge of the frequency of natural wildfires on the islands prior to human occupation. A sedimentary sequence that was recently discovered in a small canyon on San Nicolas Island contains evidence of at least 24 burn events that date to between ∼37 and 25 ka (thousands of calibrated 14C years before present), well before humans entered North America. The evidence includes abundant macroscopic charcoal, blackened sediments, and discrete packages of oxidized, reddish-brown sediments that are similar in appearance to sedimentary features called “fire areas” on Santa Rosa Island and elsewhere. Massive fine-grained sediments that contain the burn evidence are interpreted as sheetwash deposits and are interbedded with coarse-grained, clast-supported alluvial sediments and matrix-supported sands, pebbles, and cobbles that represent localized debris flows. These sedimentary sequences suggest that the catchment area above our study site underwent multiple cycles of relative quiescence that were interrupted by fire and followed by slope instability and mass wasting events. Our 14C-based chronology dates these cycles to well before the arrival of humans on the Channel Islands and shows that natural wildfires occurred here, at a minimum, every 300–500 years prior to human occupation.

Extraction of in situ cosmogenic 14C from olivine

Chemical pretreatment and extraction techniques have been developed previously to extract in situ cosmogenic radiocarbon (in situ 14C) from quartz and carbonate. These minerals can be found in most environments on Earth, but are usually absent from mafic terrains. To fill this gap, we conducted numerous experiments aimed at extracting in situ 14C from olivine ((Fe,Mg)2SiO4). We were able to extract a stable and reproducible in situ 14C component from olivine using stepped heating and a lithium metaborate (LiBO2) flux, following treatment with dilute HNO3 over a variety of experimental conditions. However, measured concentrations for samples from the Tabernacle Hill basalt flow (17.3 ± 0.3 ka4) in central Utah and the McCartys basalt flow (3.0 ± 0.2 ka) in western New Mexico were significantly lower than expected based on exposure of olivine in our samples to cosmic rays at each site. The source of the discrepancy is not clear. We speculate that in situ 14C atoms may not have been released from Mg-rich crystal lattices (the olivine composition at both sites was ~Fo65Fa35). Alternatively, a portion of the 14C atoms released from the olivine grains may have become trapped in synthetic spinel-like minerals that were created in the olivine-flux mixture during the extraction process, or were simply retained in the mixture itself. Regardless, the magnitude of the discrepancy appears to be inversely proportional to the Fe/(Fe+Mg) ratio of the olivine separates. If we apply a simple correction factor based on the chemical composition of the separates, then corrected in situ 14C concentrations are similar to theoretical values at both sites. At this time, we do not know if this agreement is fortuitous or real. Future research should include measurement of in situ 14C concentrations in olivine from known-age basalt flows with different chemical compositions (i.e. more Fe-rich) to determine if this correction is robust for all olivine-bearing rocks.