Shell Dates Research Articles

Burning effects on the soils of the Mexican Maya Lowlands: Current evidence for understanding past events

The slash-and-burn cultivation system is a millenarian practice still used in many countries today, with significant environmental impacts. However, identifying evidence of this agricultural method in soil records is challenging due to natural and human factors that can obscure or alter fire signals. This challenge is particularly evident in the Yucatán Peninsula, where the ancient Maya practiced slash-and-burn agriculture in a dynamic tropical karst environment, making it difficult to pinpoint the origins and consequences of this practice. In this study, we use a recent agricultural burning in a karstic landscape in Chiapas as a model to understand alterations in soil properties. We then apply these insights to identify similar features at a Maya site on the Yucatán Peninsula, specifically at Coyote Quarry. For both cases, we conducted micromorphological analysis, soil physical-chemical characterization, thermogravimetry and differential scanning calorimetry analysis, and radiocarbon dating of mollusk shells, soil organic carbon, and charcoal fragments (the latter exclusively at Coyote Quarry). Our findings reveal significant effects on the soils from the Chiapas site post-burning, including changes in grain size distribution, pH, and total organic and inorganic carbon. These results were compared to those obtained from the Coyote Quarry soils and pedosediments. Key findings include micromorphological features, the presence of charcoal and other burned materials, and pyrogenic carbon identified through thermal analyses. The preservation of these burned features was facilitated by the typical karstic vertical soil erosion and redeposition as pedosediments in karstic depressions. The intrinsic association of the fire signals found in these karstic sinkholes and their radiocarbon dates enables us to link this evidence to the initiation of slash-and-burn agriculture by the Maya in the Yucatán Peninsula during the transition between the Archaic and Early Pre-Classic periods.

Electron Spin Resonance Dating of Shells—A Systematic Literature Review

Electron Spin Resonance Dating of Shells—A Systematic Literature Review

The potentials and limitations of two taxa of terrestrial snails (polygyra spp. And euglandina rosea) as a source material for the radiocarbon dating of indigenous shell mounds and middens in florida, usa

Preferred materials for radiocarbon dating are often rare on coastal archaeological sites, leading many archaeologists to date more common and abundant marine shells. But marine shell is often difficult to reliably radiocarbon date owing to the effects of the marine reservoir effect, especially where local environmental factors such as hardwater and limestone substrates complicate corrections. Terrestrial gastropods provide a potential alternative material for radiocarbon dating, although they are variably impacted by the same environmental factors and thus require testing to determine their reliability for dating archaeological deposits. We compare radiocarbon dating results from paired archaeological samples of wood charcoal and Odocoileus virginianus (white-tailed deer) bone with the shells of two terrestrial snail taxa—Polygyra spp. (flat coil snails) and Euglandina rosea (rosy wolf snails)—from the Cockroach Key archaeological site, in Tampa Bay, an open-water estuary on the western coast of the Florida Peninsula, USA. In one-third of the total 12 pairings, we found no statistically significant difference at the 95% probability level. For the other pairings, differences between snail shell dates and the reference dates ranged from decades to several centuries. Overall, our results suggest that both taxa have the potential to yield reliable radiocarbon dates, although not without complications.

The Hālawa dune site, Moloka‘i, Hawaiian Islands: New excavations, redating, and new interpretations

The Hālawa dune site on Moloka‘i Island has played a key role in understanding early Hawaiian history. Initially excavated in 1969–1970 and regarded then as dating to the early phase of Hawaiian settlement, the site yielded a diverse artifact assemblage and the first evidence for round-ended house foundations in Hawai‘i. Prior attempts to refine the site’s chronology yielded inconsistent results. Renewed excavations and re-analysis of previously excavated collections from the site have allowed for recovery of a stratigraphically controlled sample of faunal materials and charcoal. AMS dating of candlenut endocarp indicates initial use of Mound A cal AD 1366–1610, with occupation continuing into the post-contact period. Matched pairs of candlenut and marine mollusk shell dates allow for the derivation of a Hālawa-specific marine reservoir value (ΔR = −267), in turn permitting the calibration of 12 dates on marine mollusk shells excavated from Mound B in 1970. Faunal materials from the 2020 Mound A excavation add new information on patterns of marine exploitation, and on a trend of increased frequency of domestic pig and dog over time. Charcoal identified from Mound A indicates largely native forest taxa, but with several Polynesian-introduced plants as well, including candlenut, breadfruit, and mountain apple.

Radiocarbon dating of small snail shells in a loess-palaeosol sequence at Mangshan, central China

In late Quaternary Chinese loess-palaeosol deposits, snail shells are often the only radiocarbon dating material available for building a chronology. However, the reliability of radiocarbon dating different small snail shells remains an open question. Here, we collected different small snail shells from a loess-palaeosol sequence located in central China to test the radiocarbon ages of snail shells. Both solid graphite target AMS (accelerator mass spectrometry) and gas AMS measurements were performed to evaluate the reliability and possible contamination of small shells of different sizes for radiocarbon dating. The 14C ages of the graphitized samples were generally consistent with the corresponding OSL ages, indicating the reliability of small-snail-shell 14C dating in Chinese loess deposits. The ages of the surface fractions of the small snail shells were close to the ages of the interior parts, and contamination after chemical treatment was limited, revealing that the fossil snail shells behaved as a closed system during burial. In addition, the gas measurement results further demonstrated the different degrees of reliability among various snail species. For minute taxa, such as Vallonia and Pupilla, their shells can mainly reveal reliable 14C ages. For larger taxa, such as Cathaica and Metodontia, much attention should be given to selecting appropriate shells. Large individuals and snail hatchlings may contain considerable amounts of old carbon, and only small shells larger than newly incubated snails (<10 mm and > 2 mg) can provide reliable 14C ages. Our study shows that the limestone effect on the radiocarbon ages obtained from most small snail shells is negligible, thus providing a great potential method to constrain the accurate ages of late Quaternary loess deposits.

Dating Marine Shell: A Guide for the Wary North American Archaeologist

AbstractRadiocarbon dates on marine shell and other materials of marine origin appear significantly older than contemporaneous samples of terrestrial/atmospheric origin. Misunderstandings regarding the mechanisms that give rise to this “marine reservoir effect” (MRE), the terminology used to define it, and the mathematics used to describe it cause many coastal archaeologists to distrust or misinterpret marine shell dates. The recent release of a reformulated 14C calibration curve for marine samples (Marine20), which necessitates recalculation of all local reservoir age corrections, may add to the confusion. Here, we review the benefits of dating shell; provide a plain-language explanation of the mechanical, chemical, biological, and cultural processes that give rise to age disparities associated with the MRE; and offer advice to archaeologists intending to date marine shell. Our hope is that these comments will not only aid archaeologists in the planning and interpretive stages of research but also assist in assessing the reliability of legacy chronologies based on marine materials. More broadly, we encourage careful evaluation of all sources of uncertainty in all 14C chronologies, whether based on terrestrial or marine materials.

Subaqueous clinoforms created by sandy wave-supported gravity flows: Lessons from the Central California shelf

Subaqueous clinoforms are an important shelf feature. Their origins are typically associated with subaerial deltas but recent work has identified similar features in settings without a significant fluvial source (Mitchell, 2012; Patruno et al., 2015; Patruno and Helland-Hansen, 2018). These other studies have shown that such subaqueous clinoforms, also known as infralittoral prograding wedges (IPWs), are created largely by wave-induced processes. This study uses geophysical, sedimentological, and radiocarbon data to determine the sedimentary characteristics and genesis of a shore-parallel subaqueous clinoform developed far from any significant river on the central California continental shelf; a sedimentary shelf buildup known locally as the Cross Hosgri Slope. Sediment cores through the clinoformal sedimentary unit reveal that it is composed of beds with an erosive base, overlain by a thin (∼ 15 cm) coarsening upward sequence of shelly fine sands transitioning to a fining upward sequence marked by alternating parallel and ripple cross laminated very fine sands. The sedimentary succession is often capped by fine silts that are commonly interbedded with thin very fine sand beds. Radiocarbon dating of shells collected just above the erosive base indicate the subaqueous clinoform initiated progradation ∼7 ka, nucleating on an older Younger Dryas relict shoreface. We suggest the Cross Hosgri Slope was created by winter-storm waves mobilizing sands in water depths up to ∼70 m that transitioned into wave-supported gravity flows. The wave-supported gravity flows traveled downslope to water depths of up to ∼80 m, corresponding to the foot of the subaqueous clinoform. They did not travel beyond this depth as wave influence at these depths is negligible and the shelf gradient is insufficient to maintain movement of the load alone. Our work suggests that wave-supported gravity flows can entrain very fine sands and silts and build subaqueous clinoforms, even in the absence of a significant river source. Furthermore, we provide a facies model for sandy wave-supported gravity flow deposits.

In both directions: Expansions of European land snails to the north and south from glacial refugia

AbstractAimWere postglacial recolonizations facilitated by persistence close to the colonized areas rather than by dispersal ability allowing for colonization from distant sources? This question is particularly relevant for organisms with low active dispersal abilities and lacking specialized propagules. Here we identified glacial refugia of four Central European land snail species, including one for which a northern glacial refugium in the region was indicated by indirectly dated fossils.LocationCentral Europe and the Balkans.TaxonPulmonate land snails: Monachoides incarnatus (Hygromiidae) and Helix pomatia, Helix thessalica and Caucasotachea vindobonensis (Helicidae).MethodsWe used continuous phylogeographic analysis of mitochondrial sequence data to trace the origin of postglacially expanding lineages. We assessed the credibility of results from our focal species, M. incarnatus, by comparison to three other broadly sympatric land snail species. In addition, we performed direct radiocarbon dating of fossil shells to verify the presumed glacial presence of M. incarnatus on the territories of Czechia and Slovakia and to test the reliability of the available fossil data.ResultsIn three of the four species, the molecular data supported refugia located south of the Pannonian Basin, from the south‐eastern Alps to the south‐western Carpathians, but not more northerly. The direct radiocarbon dating resulted in younger dates than previously assumed. However, the molecular data also revealed unexpected, yet substantial postglacial southward expansions of M. incarnatus and C. vindobonensis into the Balkans.Main ConclusionsNeither the phylogeographic analyses nor the direct radiocarbon dating provided evidence for the glacial survival of studied land snail species in Central Europe. The refugia located adjacent to the Pannonian Basin were the most important source of postglacial expansions to Central Europe, but were also the source of the expansion southwards. Both climatic factors and biological interactions might explain why such southward expansions seem rather rare in Europe.

Metal contamination in a sediment core from Osaka Bay during the last 400 years

Osaka Bay adjacent to the Kyoto–Osaka–Kobe metropolitan area was affected by severe metal pollution during the twentieth century; yet little is known about the trace metal sources and pre-industrial human activities. We have determined the elemental concentrations and zinc stable isotope ratios (δ66Zn) in bulk sediments and the trace metal concentrations in chemical fractions of a 9-m-long sediment core from Osaka Bay. Our goals were (1) to reconstruct the historical trace metal contamination, and (2) to identify anthropogenic Zn sources and the solid phases of anthropogenic trace metals. The core provided a continuous environmental record of the last 2300 years based on radiocarbon dating of molluscan shells. Copper, Zn, and Pb showed an initial enrichment from the 1670s AD, which could be caused by human activities due to an increasing population. In agreement with previous findings, the trace metal concentrations slightly increased from the 1870s, strongly increased from the beginning of the twentieth century, and peaked around 1960 before environmental pollution control laws were enacted. Increasing trace metal concentrations in the acid-labile and reducible fractions obtained by the Community Bureau of Reference (BCR) sequential extraction procedure toward the surface indicate carbonates and Mn oxyhydroxides were the primary fractions for anthropogenic trace metals. The δ66Zn values (1) were constant until the 1940s, suggesting that the average δ66Zn of industrial sources was indistinguishable from that value of the natural background, (2) showed a slight decrease from the 1950s and remained constant until the present, and (3) fell in a binary mixing process between a lithogenic (~ + 0.27‰) and an anthropogenic endmember (~ + 0.17‰), the latter likely representing a mixture of various Zn sources such as road dust, tire wear, industrial effluents, and effluents from wastewater treatment plants. We conclude the combination of Zn stable isotopes together with chemical fractions obtained by the BCR method represents a promising approach to assess the trace metal sources and their potential mobility in sediment cores from anthropogenically affected coastal areas.

Stratigraphy and eruptive history of Corbetti Caldera in the Main Ethiopian Rift

The East African Rift (EAR) hosts the highest density of peralkaline volcanoes of any region globally, making it an ideal location to study the subaerial and magmatic processes of peralkaline volcanism. Corbetti Caldera is one such peralkaline centre found within the southern part of the Main Ethiopian Rift (MER), a segment of the EAR. Corbetti is actively deforming and has previously undergone large-scale Plinian eruptions. However, our broader understanding of Corbetti's evolution is limited. Here we present a detailed study of the pre-, syn-and post-caldera eruptive deposits and compile them into a composite stratigraphy of the volcanic sequence. We find evidence for multiple previously undocumented large-scale eruptions, including the deposition of two pre-caldera lithic lag breccias, and we identify three additional post-caldera obsidian lavas. We constrain the age of a young Tuff Cone, Biftu, which sits outside the caldera walls, to <7375 ± 54 cal BP through 14C dating of shells within an associated PDC deposit. The Wendo Koshe Cone, the most recent cone within the complex, is interpreted as the remains of two cones that opened in close proximity to each other and formed the vents for two pyroclastic eruptions: Bedded Pumice and Wendo Koshe Younger Pumice (WKYP). There is abundant evidence for the occurrence of pyroclastic density currents (PDC) within the caldera, usually associated with pumice cone-forming eruptions. These flow deposits rarely extend beyond the caldera walls. From our compiled stratigraphy, we estimate a recurrence rate of one eruption per 300–400 years over the last 2.3 ky.These findings are especially significant as Corbetti is earmarked for potential geothermal exploration, and an understanding of eruptive frequency and style provides vital context to assess the potential hazards associated with this future infrastructure.

Holocene ice-free strait followed by dynamic Neoglacial fluctuations: Hornsund, Svalbard

The recession of the Hornbreen-Hambergbreen glaciers (Hornsund, Svalbard) will lead to the formation of a strait between the Greenland and Barents Seas within a few decades. We provide evidence for the earlier existence of this strait, in the Early–Middle Holocene and presumably since 1.3 ka cal. BP until glacier advance 0.7 ± 0.3 ka or earlier. Radiocarbon dating of mollusc shells from the ground moraines in the Hornbreen forefield indicate the existence of the marine environment at the contemporary glacierized head of Hornsund since 10.9 ka cal. BP or earlier due to glacier retreat. The gap in the radiocarbon dates between 3.9 and 1.3 ka cal. BP and the published results of 10Be exposure dating on Treskelen suggest the strait’s closure after glacier advance in the Neoglacial. Subsequent re-opening occurred around 1.3 ka cal. BP, but according to 10Be dates from Treskelen, the strait has again been closed since ca. 0.7 ± 0.3 ka or earlier. The oldest known surge of Hornbreen occurred around 1900. Analysis of Landsat satellite images, morphometric indicators characterizing the glacier frontal zones and previous studies indicate one surge of Hambergbreen (1957–1968) and five re-advances of Hornbreen in the 20th century (after 1936, between 1958 and 1962, in 1986–1990, 1998–1999, 2011). While the warmer Holocene intervals might be a benchmark for the effects of future climate change, glacier dynamics in post-Little Ice Age climate warming seems to be an analogue of glacier retreats and re-advances in the earlier periods of the Holocene.

Recent Surface Rupturing Earthquakes along the South Flank of the Greater Caucasus near Tbilisi, Georgia

ABSTRACTFault characterization is a critical step toward improving seismic hazard assessment in the Georgian Greater Caucasus but is largely absent from the region. Here, a paleoseismic trench near the capital city of Tbilisi revealed evidence for recurring surface rupture on a shallowly north-dipping thrust fault. The fault has broken through the overturned forelimb of a fault-propagation anticline that folds a sequence of soils and deposits. Stratigraphic relationships and radiocarbon dating of terrestrial gastropod shells corrected for “old carbon” age anomalies loosely constrain three surface-deforming earthquakes on this fault between ∼40 and ∼3 ka, with variable dip-slip displacements ranging between 0.35 and ∼3 m, and a cumulative displacement of 6.5 ± 0.85 m. Single event slips and recurrence intervals (11, 25, and 3 ka open interval) at this site demonstrate apparent slip rate variations of 3−7× over the last two earthquake cycles on the fault, which we attribute to possible rupture complexity involved in crustal thrust fault earthquakes. This study provides a structural and geochronologic template for future paleoseismic investigations in the Greater Caucasus while highlighting some of the challenges of conducting seismic source characterization in this region.

Radiocarbon Dating of the Nyixoi Chongco Rock Avalanche, Southern Tibet: Search for Signals of Seismic Shaking and Hydroclimatic Events

Landslides are important agents of the surface processes involved in the growth of mountainous topography. Dating prehistoric landslides is a prerequisite for establishing the relationships between prehistoric slope instability, and past climatic regimes and paleoseismic records. The Nyixoi Chongco rock avalanche (NCRA) is located in the Angang graben within the N–S trending rift zone in southern Tibet. It represents a giant prehistoric mass wasting event that was characterized by exceptional mobility and a large volume. However, the exact emplacement time and origin of the NCRA are still controversial. In this study, we conducted 14C dating of peat layers and snail shells to constrain the emplacement age of the NCRA. The 14C ages of the organic material and plant remnants in the basal peat layer are 1272–1389 and 1299–1404 cal AD, respectively. The 14C ages of aquatic snail shells and the bog overlying the rock avalanche are 425–565 and 1022–159 cal AD, respectively. These results indicate that the NCRA consisted of at least two separate and distinct events, instead of the single event suggested by previous studies. Based on field investigations and temporal correlations, we infer that there may be no paleoseismic records in the Angang graben that would corroborate a coseismic trigger for the NCRA. Therefore, we suggest that the 14C ages of the sediments below and above the landslide rocks should be interpreted carefully. The 14C ages alone do not provide sufficient evidence to infer the true trigger of the NCRA event.

CLARIFYING THE AGE OF INITIAL SETTLEMENT HORIZON IN THE MARIANA ISLANDS AND THE IMPACT OF HARD WATER: A RESPONSE TO CARSON (2020)

ABSTRACTThis paper is a response to criticism by Carson (2020) concerning the age of the Unai Bapot archaeological site in the Mariana Islands. Of specific contention are supposed errors in the marine radiocarbon (14C) research reported by Petchey et al. (2017). According to Carson, this work produced marine reservoir offsets (ΔR) of “suspiciously wide variance from each other … and those results were incompatible with the site’s reported stratigraphy and dating (Carson 2008) as well as with other previously calculated marine reservoir corrections in the Mariana Islands (Carson 2010: 3).” This statement is misleading on all three points. Our reply discusses the problems encountered when dating shells from near-shore “marine” environments where terrestrial input, in particular hard water, may complicate date interpretation. We outline a cautionary tale relevant to any researcher using “marine” shell dates to develop regional archaeological chronologies in regions dominated by limestone.

A chronology of post-glacial mass-transport deposits on the Canadian Beaufort Slope

Extent and chronology of 24 buried and seabed-exposed mass transport deposits (MTDs) on the continental slope of the Canadian Beaufort Sea were compiled towards a regional geo-hazard assessment of the Beaufort region. A total of 2220 lines of 3.5 kHz sub-bottom profiler (SBP) data (~40,000-line kilometres) covering an area of 9740 km2 were analyzed to allow a new understanding of slope instability in the region. Several sediment cores acquired across the region allow dating of the sediment cover. A regionally representative seismic-stratigraphy (type section, ca. 60 m thick) of the stratified sediments mantling the slope was defined. The MTDs all occur above a marked change in sedimentation style from a deeper-situated slope fan that varies substantially laterally to the above-lying stratified layers that host the MTDs. The type section comprises three sedimentary units bound by two prominent markers. Relative ages for the MTDs were defined by measuring the thickness of sediments overlying each MTD and linking that sediment package to the type section. Two 3-D seismic data volumes across the study region verified interpretations from 2D data by imaging internal deposit character and down-slope continuity of the MTDs. Seismic amplitude and similarity attributes enabled identification of further MTD events, incompletely imaged by the SBP data. A composite chronostratigraphy based on 14C dating of foraminifera and shells was assembled despite the fact that the type section is far thicker than sediment coring limits. Sub-unit thickness varies up- and down-slope such that a selection of cores across the study region enabled its compilation. The marked change in sedimentation style at the base of the type section required substantial extrapolation to date. Simple (quadratic function) age models project that the base of the type section signals the end of the last glacial maximum (Wisconsinan, or marine isotope stage 2 glaciation) and initiation of pro-glacial plume sedimentation (the deglacial), and finally post-glacial (marine) ultimately from the Mackenzie River. The MTD abundance above this contact defines an average theoretical recurrence rate of one MTD per ~1000 yrs.; however, MTDs are clustered temporally with the highest number of events occurring just prior to the onset of the Younger Dryas at around 13 ka BP (cal.).

Early Holocene phytolith records for three shell midden sites, Yongjiang River, Guangxi Province, China

Phytolith records from three proximal freshwater shell midden sites document plant exploitation and local palaeoecological changes during the early Holocene in present-day Nanning City, Guangxi Province, China. Radiocarbon dating of freshwater gastropod shells indicates the midden sites formed sequentially, with variable chronological overlaps: Baozitou at 12,000-11,130 cal BP, Shichuantou at 11,250-10,500 cal BP and Nabeizui at 11,060-9560 cal BP. Palaeoecological data are inferred to show variable trends of forest reduction or recovery during the formation of each midden, suggesting different intensities of site use and impacts upon the local environment. Significantly, palms (Arecaceae) became more frequent during the middle period of formation at each site, which is interpreted to represent resource intensification and selective exploitation of palms by the communities who created each midden.

Vital evidence: Change in the marine 14C reservoir around New Zealand (Aotearoa) and implications for the timing of Polynesian settlement

Precise and accurate radiocarbon chronologies are essential to achieve tight chronological control for the ~ 750-years since Polynesian settlement of New Zealand. This goal has, however, been elusive. While radiocarbon datasets in the region are typically dominated by marine and estuarine shell dates, such chronological information has been ignored by those interpreting the timing of key events because a detailed regional calibration methodology for marine shell, comparable to the highly precise Southern Hemisphere calibration curve, is lacking. In this paper, we present the first temporal 14C marine offset (ΔR) model for New Zealand based on paired estuarine/marine and terrestrial radiocarbon dates from 52 archaeological contexts. Our dataset displays significant offsets between the measured New Zealand data and the modelled global marine radiocarbon curve. These shifts are associated with oceanographic fluctuation at the onset of the Little Ice Age ~ AD 1350–1450 (650–500 BP). The application of a regional and temporal correction to archaeological shell dates provides complimentary information to terrestrial radiocarbon production and has the potential to add structure to the blurred chronology that has plagued archaeological theories about the colonization of New Zealand, and other Pacific islands, for decades.

Chronostratigraphy of two Late Pleistocene loess-palaeosol sequences in the Rhône Valley (southeast France)

A sedimentological and chronostratigraphical investigation was carried out on two loess sections located in the Mediterranean area in southeast France along the Rhône River (Lautagne) and the lower reach of a tributary of the Rhône River (Collias). High-resolution sampling (5–20 cm) for magnetic susceptibility, grain size distribution (including non-parametric end-member modelling), colour reflectance and geochemistry was performed. The chronology was based on luminescence dating of quartz grains and radiocarbon dating of small gastropod shells, coupled with hierarchical Bayesian modelling. The Collias section (∼8 m thick) records the whole last climatic cycle. It comprises a thick red basal pedocomplex S1 developed during the Last Interglacial and the Early Glacial, similar to that observed elsewhere in southern and southeastern Europe. Loess deposition occurred during the Lower (L1L2) and the Upper Pleniglacial (L1L1). It was interrupted by soil formation during the Middle Pleniglacial, of which a brown Bwk horizon has been preserved (L1S1). By contrast, the ∼5 m thick Lautagne section provides a detailed record of the Upper Pleniglacial. Weakly developed hydromorphic soils are correlated with the Greenland Interstadials GI-4 to GI-2, while the main period of coarse loess sedimentation corresponds to the Greenland Stadials GS-5 to GS-2. At a regional scale, the time of loess deposition ranges between 38.5 ka and 12 ka, with a peak at ∼28–24 ka, overlapping with the maximal advance of the Alpine Ice Sheet (AIS). This strongly suggests that regional glacier dynamics was the main driver of loess sedimentation.

Multi-proxy approach to unravel methane emission history of an Arctic cold seep

Arctic Ocean sediments contain large amounts of methane in the form of free gas and gas hydrate. This highly dynamic methane reservoir is susceptible to be modified by bottom water warming. The warming may lead to gas hydrate destabilization releasing elevated methane fluxes to the seafloor and seawater. Reconstructing past methane dynamics can be achieved by using specific proxies left in the geological record. In this study, we apply a multi-proxy approach for paleo seepage reconstruction from sediment records at gas hydrate mounds (GHMs) in Storfjordrenna (south of the Svalbard archipelago). These shallow water (∼380 m water depth) systems are potentially vulnerable to global warming related temperature changes. 14C dating of foraminifera shells indicated an onset of deglaciation in the Storfjordrenna region at ∼20 kyr BP and allowed us to establish a stratigraphic context based on sediment Zr/Rb and Fe/Ca ratios. Several major (between 15 and 17 kyr BP) and minor methane venting phases were identified and interpreted to be related to gas hydrate instability triggered by isostatic adjustment right after the onset of the deglaciation. The detection of all major methane releases was only possible by combining data sets of stable carbon isotope compositions of foraminifera, mineralogy and δ13C values of authigenic carbonates, and abundance and stable carbon isotope signatures of lipid biomarkers. The most robust single proxy in this study was provided by the δ13C values of archaeal biomarkers. In contrast, the sediment Ba/Ti ratios recorded only the major events. Our results highlight the complexity and heterogeneity of methane dynamics in a small area of some hundred meters across.

A 50,000-year record of lake-level variations and overflow from Owens Lake, eastern California, USA

A continuous lake-level curve was constructed for Owens Lake, eastern California by integrating lake-core data and shoreline geomorphology with new wind-wave and sediment entrainment modeling of lake-core sedimentology. This effort enabled refinement of the overflow history and development of a better understanding of the effects of regional and global climate variability on lake levels of the paleo-Owens River system during the last 50,000 years. The elevations of stratigraphic sites, plus lake bottom and spillway positions were corrected for vertical tectonic deformation using a differential fault-block model to estimate the absolute hydrologic change of the watershed-lake system. New results include 14C dating of mollusk shells in shoreline deposits, plus post-IR-IRSL dating of a suite of five beach ridges and OSL dating of spillway alluvial and deltaic deposits in deep boreholes. Geotechnical data show the overflow area is an entrenched channel that had erodible sills composed of unconsolidated fluvial-deltaic and alluvial sediment at elevations of ∼1113–1165 m above mean sea level. Owens Lake spilled most of the time at or near minimum sill levels, controlled by a bedrock sill at ∼1113 m. Nine major transgressions at ∼40.0, 38.7, 23.3, 19.3, 15.6, 13.8, 12.8, 11.6, and 10.6 ka reached levels ∼10–45 m above the bedrock sill. Several major regressions at or below the bedrock sill from 36.9 to 28.5 ka, and at ∼17.8, 12.9, and 10.4–8.8 ka indicate little to no overflow during these times. The latest period of overflow occurred ∼10–20 m above the bedrock sill from ∼8.4 to 6.4 ka that was followed by closed basin conditions after ∼6.4 ka. Previous lake core age-depth models were revised by accounting for sediment compaction and using no reservoir correction for open basin conditions, thereby reducing discrepancies between Owens Lake shoreline and lake-core proxy records. The integrated analysis provides a continuous 50 ka lake-level record of hydroclimate variability along the south-central Sierra Nevada that is consistent with other shoreline and speleothem records in the southwestern U.S.