In the San Luis Obispo Bay area of central California, interpretations of marine terrace ages have been hampered by inconsistent results from geochronological indicators (U-series ages of corals and correlations using amino acid racemization of mollusks) and seemingly contradictory paleozoogeographic aspects of fossil faunal assemblages. New U-series ages of corals, amino acid data from mollusks, and detailed analyses of fossil mollusk assemblages are presented that reconcile many of the apparently discordant results from previous studies. The two lowest-elevation terraces are Q1 (lower) and Q2 (upper); both are thought to date from high-sea stands of marine isotope stage (MIS) 5, the last interglacial complex. A combination of U-series dating and amino acid racemization results indicates that the Q1 terrace probably dates to the-80 ka high-sea stand of MIS 5a. U-series analyses of corals from the Q2 terrace show open-system histories, but consideration of two alternative open-system histories indicates that this terrace likely hosts corals dating to the high-sea stands of MIS 5c (-105-95 ka) and MIS 5e (-130-115 ka). Amino acid ratios in the fossil bivalve Leukoma staminea (Conrad, 1837) support the age differences between the two terraces and the open-system models suggested by the Useries data. Alkenone studies along much of the coast of California have shown that sea surface temperatures (SSTs) of the eastern Pacific Ocean were warmer than present during MIS 5e. During MIS 5c, on the other hand, SSTs were either similar to the present or cooler, but in either case, much cooler than during MIS 5e. The paleozoogeographic aspects of the molluscan faunas from the Q2 terrace support an interpretation that these deposits contain a mixture of fossils from both the MIS 5c and 5e high-sea stands. Fossils from the Q2 terrace include southern or southward-ranging species (likely dating to MIS 5e) and northern or northward-ranging species (likely dating to MIS 5c). A mixture of MIS 5e and MIS 5c fossils on the Q2 terrace is consistent with similar data reported for other localities in California. This phenomenon is explained by glacial isostatic adjustment (GIA) processes, which brought about a higher-than-present sea level during MIS 5c. GIA processes, combined with a low uplift rate, allowed mixing of MIS 5e fossils during the MIS 5c high-sea stand through capture of an MIS 5e terrace that had experienced minimal uplift in-20,000 years. Terrace reoccupation and fossil mixing such as that in the San Luis Obispo Bay area can be expected along any part of the Pacific Coast of North America that was affected by GIA processes and where uplift rates are low.

In several early studies, central California marine terraces between Santa Barbara and Point Conception were interpreted to record sea-level high stands of the last interglacial complex, similar to 80 ka to similar to 120 ka (marine isotope stage [MIS] 5). These ages and their elevations (similar to 20 m to similar to 45 m) indicate modest rates of tectonic uplift, similar to those from other localities in southern and central California. A recent study, using a combination of luminescence and radiocarbon dating, has challenged the older age interpretations, implying much younger terrace ages, between similar to 40 ka and similar to 55 ka (MIS 3). From these new ages and a considerably lower sea level during MIS 3, much higher rates of tectonic uplift are inferred. In the present study, new uranium-series ages of terrace corals and amino acid age estimates of terrace mollusks were determined to test these competing interpretations. With the exception of a low-elevation terrace in Isla Vista (near Santa Barbara) that dates to MIS 3, terraces farther west are interpreted to date to MIS 5 and imply tectonic uplift rates of 0.20-0.34 m/kyr. A compilation of data for the region yields a decreasing rate of late Quaternary uplift from east, near Ventura, to west, near Point Conception. This trend is interpreted to reflect a decreasing influence of the processes of compression and crustal shortening south of the Big Bend in the San Andreas fault.

Discovery of human footprints in alluvium dated to the Last Glacial Maximum (LGM) at White Sands, New Mexico, was a notable step in understanding the initial peopling of the Americas, but that work was met with criticism focused on the reliability of the materials used in the radiocarbon dating (seeds of Ruppia and pollen). This paper reports on an independent study of the chronology of a previously unrecognized stratigraphic record of paleolake Otero that is directly traceable into the track-bearing alluvium. The stratigraphic data along with 26 additional radiocarbon dates on palustrine mud determined by two labs independent of the original investigations document an aggrading lake/wetland/stream record that includes the tracks and spans >23.6 thousand years to ~17.0 thousand calibrated years before present, providing another line of evidence further supporting the validity of an LGM age for the tracks.

The bulk organic-matter content of near-surface sediment is widely used for radiocarbon (C-14) dating, despite often containing organic carbon (OC) older than the depositional age. Low-temperature combustion can mitigate the influence of old OC, producing ages closer to the depositional age. We developed a simple method to determine the C-14 age of the low-temperature (<250 degrees C) component of bulk sediment. Sediment samples from five Arctic lakes were heated up to 400 degrees C, revealing that at 250 degrees C, about half of the OC combusts, leaving behind the more recalcitrant fraction. We applied this method to 64 samples from late glacial and Holocene sediment cores, analyzing C-14 and % OC in two aliquots: one heated at 250 degrees C and one unheated. The low-temperature C-14 age was calculated by difference using a two-component mixing model. Accuracy was assessed by comparing ages with macrofossils from 48 samples, and reproducibility was tested using a standard reference material. Results show that low-temperature combustion yields C-14 ages with an interquartile range of 115 years, and with reproducibility on par with that of macrofossil dating. On average, the ages differ by 932 years from macrofossils, compared to a 2425-year difference for conventional bulk-sediment ages analyzed in this study. Accuracy improves for samples where the proportion of residual OC after heating is low. This practical and efficient method complements macrofossil dating, supports analysis of a large number of samples, and provides insights into sedimentary carbon cycling.

Bioerosion is a valuable tool for inferring palaeoenvironmental and palaeoclimatic changes over time and across different regions. However, studies of bioerosion traces are scarce in the Southern Hemisphere. Most ichnological studies within Argentina are concentrated in San Jorge Gulf (Patagonia, Argentina) and little is known about deposits located north of the Gulf. Here, we focus on bioerosion traces on Quaternary mollusc shells. Samples were collected from Quaternary marine deposits at the Bah & iacute;a Vera-Cabo Raso sites in northern San Jorge Gulf. To resolve age discrepancies reported in the literature, we use amino acid racemization and radiocarbon dating to confirm the presence of beach ridge deposits from Marine Isotope Stage (MIS) 5 and MIS 1. Fourteen ichnotaxa are recorded in the study area. Additionally, distinct variations in the pattern of bioerosion across different ages are observed, indicating that environmental changes occurred in the northern San Jorge Gulf between the MIS 5 interglacial and the Holocene. This reinforces the hypothesis that there is an association between bioerosion, productivity and circulation in the Southern Atlantic Ocean.

Northern Arizona University, Flagstaff, Arizona, USA, recently installed a MIni CArbon DAting System (MICADAS) with a gas interface system (GIS) for determining the C-14 content of CO2 gas released by the acid dissolution of biogenic carbonates. We compare 48 paired graphite, GIS, and direct carbonate C-14 determinations of individual mollusk shells and echinoid tests. GIS sample sizes ranged between 0.5 and 1.5 mg and span 0.1 to 45.1 ka BP (n = 42). A reduced major axis regression shows a strong relationship between GIS and graphite percent Modern Carbon (pMC) values (m = 1.011; 95% CI [0.997-1.023], R-2 = 0.999) that is superior to the relationship between the direct carbonate and graphite values (m = 0.978; 95% CI [0.959-0.999], R-2 = 0.997). Sixty percent of GIS pMC values are within +/- 0.5 pMC of their graphite counterparts, compared to 26% of direct carbonate pMC values. The precision of GIS analyses is approximately +/- 70 C-14 yrs to 6.5 ka BP and decreases to approximately +/- 130 C-14 yrs at 12.5 ka BP. This precision is on par with direct carbonate and is approximately five times larger than for graphite. Six Plio-Pleistocene mollusk and echinoid samples yield finite ages when analyzed as direct carbonate but yield non-finite ages when analyzed as graphite or as GIS. Our results show that GIS C-14 dating of biogenic carbonates is preferable to direct carbonate C-14 dating and is an efficient alternative to standard graphite C-14 dating when the precision of graphite C-14 dating is not required.

The South Kenya Rift is comprised of a series of N-S-oriented grabens with sediments that preserve an approximate one-million-year environmental history that reflects the interplay of climate, tectonism and volcanism. This study attempts to disentangle the relative roles of these major controls by comparing the geochemical records preserved in three sedimentary basins. The study focuses on the Koora Basin using bulk geochemical data in a 139-m-long core. This record is then compared with geochemical data and environmental histories from a 196-m-long core at Magadi and outcrops in the Olorgesailie Basin. Four climatic phases (1000-850; 850-470; 470-400; 400-0 ka) are recognised at Koora, which can also be distinguished in the Magadi and Olorgesailie Basins. However, inter-basin contrasts also suggest that additional, non-climatic factors influenced these geochemical histories, particularly during four intervals. These include 1) the Magadi Transition (MT; -770-700 ka), 2) the Magadi Tectonic Event (MTE; -540 ka), 3) the Koora Instability Period (KIP; -325-180 ka), and 4) the Trona Precipitation Period (TPP; -105-0 ka). Prior to the MT, Zr/TiO2, La/Lu, Mo, As, V and Na/Ca in Magadi and Koora cores were similar but afterwards diverged. Major reductions in transition metals at Magadi during the MTE reflect tectonically-induced cross-rift drainage diversion. This contrasts with the Koora and Olorgesailie basins where these metals were constant from -1000 to 300 ka. The KIP represents a significant increase in volcanic inputs to the Koora Basin and increased geochemical variability. Bromine (Br), which reflects peralkaline volcanic activity and/or evaporative concentration, is elevated during the KIP at Koora but is below detection limits in the rest of the Koora core. Br in the Magadi core does not correlate with that in the Koora record, suggesting contrasting accumulation processes. The TPP represents a phase of trona precipitation at Magadi but not at Koora. This difference partly reflects increased magmatic CO2 rising along faults in the Magadi basin during a period of increasing aridity. Rare-earth element patterns indicate a major change at Magadi with many anomalies after about 325 ka to the present, caused by the development of hypersaline waters, which did not occur at Koora or Olorgesailie. The geochemical data from the three basins help to partially separate climatic controls from those related to volcanism, tectonism and local geomorphology.

Geological indicators of past relative sea level changes are fundamental to reconstruct the extent of former ice sheet during past interglacials, which are considered analogs for future climate conditions. Four interglacials, dating from Holocene to Pliocene, have left sea-level imprints in the proximity of the coastal town of Camarones in Central Patagonia, Argentina. Sea-level index points were preserved as beach ridges deposited by storm waves above modern sea level. We used highly accurate survey techniques to measure the elevation of these deposits. Satellite-derived wave measurements and wave runup models were then employed to calculate their indicative meaning (i.e., their elevation with respect to sea level at the time of deposition). The paleo relative sea levels (i.e., uncorrected for post-depositional vertical land motions) associated with the four interglacials (with 1σ uncertainties) are 6±1.5 m (Holocene); 8.7±2.1 m (MIS 5e); 14.5±1.5 m (MIS 9 or 11); and 36.2±2.7 m (Early Pliocene). Ages have been obtained using both published (U-series, Electron Spin Resonance, and Radiocarbon) and new (Amino Acid Racemization and Radiocarbon) dating constraints. We compare our results with published glacial isostatic adjustment and mantle dynamic topography predictions, and we highlight that refining these models before calculating the global mean sea level for the interglacials mentioned above is necessary. Our high-resolution data provide a significant benchmark for paleo relative sea-level studies in the Southwestern Atlantic.

The integration of the Colorado River and the Gulf of California provides critical tectonic, palaeoceanographical, and palaeontological constraints on the evolution of southeastern California and northwestern Mexico since the late Miocene. The Wind Caves Member of the Latrania Formation, Split Mountain Gorge, southern California, records the arrival of distinctive Colorado River sand to the Gulf of California. Initial interpretations suggest a similar to 5.3 Ma integration event, based on palaeomagnetic interpretations and the last occurrence of Amphistegina gibbosa roughly 10 stratigraphical metres below the first appearance of Colorado River sand. Amphistegina gibbosa is described as having a Last Occurrence Datum (LOD) in the Pacific of 5.3 Ma, coincident with the Miocene-Pliocene boundary. Numerous accounts document A. gibbosa in the eastern Pacific and Gulf of California during the Pliocene, rendering the alleged 5.3 Ma LOD as problematic. The A. gibbosa tests at Split Mountain Gorge are reworked, limiting their biochronological utility. Radiometric dating of detrital zircons and detrital sanidine grains at Split Mountain Gorge place the river integration event between 4.8 Ma and 4.6 Ma, consistent with age constraints for the arrival of the Colorado River to the lower reaches of the lower Colorado River corridor, upstream from the Gulf, by 4.8 Ma. The apparent conflict between a 5.3 Ma and a <4.8 Ma river integration event can be resolved by recognizing that the original A. gibbosa-based placement of the Miocene-Pliocene boundary at Split Mountain Gorge is untenable. The Colorado River arrived at the Gulf of California in the early Pliocene.

Paleoclimate records across the Intermountain West region of North America show significant regional variation in timing and magnitude of wet conditions that accompanied the last glacial-interglacial transition. To understand the climate controls on paleohydrologic change, well-dated records are needed across the region. The Plains of San Agustin (New Mexico, USA) is a closed-basin watershed of the American Southwest influenced by both winter westerly and summer North American Monsoon precipitation. The flat valley floors of the Plains contain lake and groundwater discharge deposits that record multiple periods of past wet climate. We present a record of hydroclimate for the past 26,000 years based on radiocarbon, U–Th series, and OSL dating of these deposits and stratigraphic correlation across the three sub-basins of the lake system. We find that two major lake oscillations occurred, coincident with the global Last Glacial Maximum (∼23–18 ka) and with Heinrich Stadial 1 (∼17–14 ka). The LGM lake cycle created a deep lake in the lowermost sub-basin, fed by marsh/lake overflow in the upper sub-basins. The Heinrich Stadial 1 wet interval attained the highest recorded lake level between ∼17.0 and 15.3 ka, merging the lower two sub-basins into one lake. Both lake cycles agree well in timing and magnitude with other lake-based records from the southwestern U.S., supporting previous interpretations that a south-shifted cool season storm track brought a dipole-like pattern of enhanced moisture to the southwest at the expense of drier conditions in the northwest during the LGM and deglaciation. A transition from lake to groundwater discharge conditions followed during the Bølling-Allerød (14.7–12.9 ka) across the study area. Wet meadows prevailed in the lower sub-basin during the Younger Dryas (12.9–11.7 ka), with marsh and open lake conditions in the upper and middle sub-basins, respectively. During the early Holocene, discrete wet intervals are recorded by ages for wet meadow deposits in all sub-basins with centennial-millennial frequency at 9.9, 8.8, 8.2 ka. These events agree well with other Intermountain West records showing wetter-than-present conditions prior to 8 ka. Two additional wet periods, centered at 6.8 and 5.4 ka, occurred during the driest known interval of the middle Holocene, and likely were partly supported in the Plains of San Agustin by summer moisture associated with the peak strength of the North American Monsoon. Our record highlights that both winter and summer moisture support water resources in New Mexico watersheds.

AbstractDeep Springs Valley (DSV) is a hydrologically isolated valley between the White and Inyo mountains that is commonly excluded from regional paleohydrology and paleoclimatology. Previous studies showed that uplift of Deep Springs ridge (informal name) by the Deep Springs fault defeated streams crossing DSV and hydrologically isolated the valley sometime after eruption of the Pleistocene Bishop Tuff (0.772 Ma). Here, we present tephrochronology and clast counts that reaffirms interruption of the Pliocene–Pleistocene hydrology and formation of DSV during the Pleistocene. Paleontology and infrared stimulated luminescence (IRSL) dates indicate a freshwater lake inundated Deep Springs Valley from ca. 83–61 ka or during Late Pleistocene Marine Isotope Stages 5a (MIS 5a; ca. 82 ka peak) and 4 (MIS 4; ca. 71–57 ka). The age of pluvial Deep Springs Lake coincides with pluvial lakes in Owens Valley and Columbus Salt Marsh and documents greater effective precipitation in southwestern North America during MIS 5a and MIS 4. In addition, we hypothesize that Deep Springs Lake was a balanced-fill lake that overflowed into Eureka Valley via the Soldier Pass wind gap during MIS 5a and MIS 4. DSV hydrology has implications for dispersal and endemism of the Deep Springs black toad (Anaxyrus exsul).

We investigated the relationship between aspartic acid d:l ratios and otolith-derived age estimates in Gulf of Mexico red snapper, Lutjanus campechanus (ages 1–26 years; R2 = 0.89) and Caribbean yellowtail snapper, Ocyurus chrysurus (ages 2–17 years; R2 = 0.84). The estimated racemization rate was 0.61 × 10−3 year−1 for red snapper and 1.28 × 10−3 year−1 for yellowtail snapper, reflecting temperature differences between study regions. Mean jackknifed error in ages predicted from aspartic acid d:l was 1.70 ± 0.39 years for red snapper and 1.57 ± 0.41 years for yellowtail snapper. Results suggest amino acid racemization may be an effective tool for direct age estimation and potentially age validation in fishes.

New detrital sanidine geochronology from the Bouse Formation in Blythe basin supports recent research that redefined the age of lower Colorado River integration. Sanidine dating along with magnetostratigraphy indicates that the Colorado River arrived at Cottonwood Valley after 5.24 Ma and reached the proto–Gulf of California between 4.80 and 4.63 Ma. Earlier studies had proposed that initial integration to the evolving proto–Gulf of California occurred at 5.3 Ma; this was based largely on magnetostratigraphy tied to ~2 Ma old ashes ~4 km higher in the section than the deposits containing the first Colorado River sand. The discrepancy in ages was ascribed to duplication of section across newly identified faults in the Fish Creek-Vallecito basin. Detrital sanidine dating from near the first arriving Colorado River sand in this basin placed a much tighter constraint on the timing of integration than was previously available. The new detrital sanidine results reported here come from near the base of a thick section of siliciclastic Bouse Formation in the northern Blythe basin, which was the last in a series of basins that the Colorado River filled before spilling into the proto–Gulf of California. New single grain sanidine ages indicate a maximum depositional age of 4.8 ± 0.1 Ma for this section. This age should approximately correspond with the first arrival of Colorado River sediment to that basin. Previous dating of single sanidine grains from the Bouse Formation indicated a similar age; currently 6 grains have been dated at younger than 5.3 Ma with increasing precision. Because the Colorado River had to pass through this basin prior to reaching the proto-Gulf of California, deposition should predate not postdate integration, supporting the new younger age. These new results and previous work are consistent with downward integration of the Colorado River through a series of closed basins in the lower Colorado River corridor, culminating in integration to the proto–Gulf of California between 4.8 and 4.63 Ma. During and after integration, the Colorado River quickly incised through sediment-filled basins and the liberation of sediment from throughout the entire system led to deposition of hundreds of meters of Bullhead Al­luvium starting around 4.6 Ma as the river adjusted to its lower base level.