Wet Intervals Research Articles

An arid phase in the Internal Dinarides during the early to middle Miocene: Inferences from Mg-clays in the Pranjani Basin (Serbia)

Lacustrine strata record long-term variations in climatic and tectonic perturbations during orogen evolution. Here we use the sedimentological record of the early to middle Miocene intramontane lacustrine Pranjani Basin (Serbia) to study tectonic and climatic variations during the evolution of the Dinaridic orogen. Our study focuses on a Mg-clay-bearing succession that belongs to the second depositional cycle. A detailed sedimentological, mineralogical and geochemical study shows that the investigated Mg-clays bearing succession consists of an alternation of smectite- (± analcime or sepiolite), dolomite-sepiolite and sepiolite layers. We interpret this high-order cyclicity to have been driven by variable water availability, i.e. by the interplay between runoff, groundwater influx and evaporation. Furthermore, we relate these pulses in water variability to wet and dry intervals under semi-arid to arid climatic conditions. Dry intervals produced a negative water balance promoting the deposition of sepiolite - and dolomite - dominated facies. Wet intervals and a positive water balance stimulated higher terrestrial input and thus favored the deposition of smectite-dominated facies. Our new data combined with a review of previous studies reveal longer-term basin-wide lake level and chemistry fluctuations, which we relate to closed and open lake phases. We suggest that these phases can be explained by alternating humid and arid phases superimposed on tectonically induced uplift and subsidence. The arid phases led to lake shrinking and deposition of an evaporite-bearing succession. During the intervening humid phase, the lake expanded. In contrast to the lakes formed in the external part of the Dinarides, the lakes that existed in its internal part contain evaporite-bearing successions, indicating that this arid phase was regionally extensive and possibly orographically controlled.

Environmental controls over Holocene carbon accumulation in Distichia muscoides-dominated peatlands in the eastern Andes of Colombia

The long-term carbon (C) and vegetation dynamics of tropical, high-Andean cushion peatlands are poorly understood. Here, we present radiocarbon-dated paleoecological records and modern microclimate data from high-elevation peatlands currently dominated by the cushion plant Distichia muscoides in the páramo (alpine tundra) of the eastern Colombian Andes. Focusing on a well-dated 4300-year-old peat core collected at 4194 m elevation from the Sierra Nevada del Cocuy, we investigated the response of peatland vegetation to hydroclimate changes and environmental controls on C accumulation. In addition, we carried out a regional synthesis of existing C accumulation data from northern Andean peatlands to place our new results in broad geographic and temporal context over the last 17,000 years. Our results show that the current Distichia-dominated peatlands are a recent landscape feature on El Cocuy occurring only during the last ∼250 years. The Distichia peatlands have extremely high sediment accumulation rates of up to 1 cm yr−1 and C accumulation rates between 183.7 and 849.5 g C m−2 yr−1, decreasing with the elevation of a site. These rates far exceed an expected modern C accumulation rate of 110 g C m−2 yr−1, calculated by extrapolating an exponential function from a late Holocene average C accumulation rate of 48.6 g C m−2 yr−1 for the northern Andes. This suggests that recent Distichia establishment represents an acceleration of C accumulation at a rate higher than expected from decomposition-related autogenic effects. Before the modern climate warming period, our record indicates that Distichia cushions have only grown intermittently, often after the peatland was flooded with mineral sediments. Mineral sediment deposits at 3290, 2590, 1620, 930, and 860 cal. yr BP correspond with wet intervals from lake-derived records of hydroclimate change in the northern Andes and indicate increases in precipitation and subsequent in-wash of mineral sediments. However, within the last 150 years, mineral sediment deposition has increased in frequency, likely due to warming-induced melting of permanent snow and ice mobilizing exposed fluvio-glacial sediments. Increased mineral nutrients, rising temperatures, and strong diurnal temperature contrast in the tropical mountains may explain the rapid C accumulation rates. Taken together, we show that Andean peatlands have experienced the largest shift in vegetation and C accumulation rates in the last 250 years than at any other time during the last 4000 years.

African Humid Period Precipitation Sustained by Robust Vegetation, Soil, and Lake Feedbacks

AbstractThe African Humid Period (∼11,000–5,000 years before present) was the most recent of several precessionally paced wet intervals during which an increase in the Northern Hemisphere summer incoming solar radiation intensifies the West African Monsoon leading to dramatic changes over northern Africa. However, insolation anomaly alone is not sufficient and feedbacks are essential for further amplification of the monsoon. The most significant feedbacks derive from the land surface, arising from changes to vegetation, soil properties, and distribution of surface water. We show that in contrast to previous studies that have explored the individual impacts of these feedbacks, a modern climate model yields a much greater increase in precipitation in response to their collective effect. Agreement with proxies is improved while the desert‐steppe transition is pushed further northward than in any previous study. In the West African Sahel, intensities of summer daily mean and extreme precipitation increase by 150% and 90%, respectively.

Droughts, flooding events, and shifts in water sources and seasonality characterize last interglacial Levant climate

Modern observations document increased drought frequency together with more intense precipitation and flooding in the world’s semi-arid and arid regions as a consequence of the warming climate. Climate models predict that such conditions will intensify in the future, impacting millions of people. Paleoclimate studies can complement the short modern observational record and model projections by documenting climate changes in the past. Here we report major shifts in the geographic sources, intensity, and seasonality of Eastern Mediterranean precipitation during the unusually warm last interglacial period Marine Isotope Stage (MIS) 5e, reflecting global shifts in the rain and desert belts, based on 234U/238U-ratios in mineral precipitates in the Dead Sea, combined with evidence from climate model simulations.In the Dead Sea catchment 234U/238U ratios are indicators of water sources, where the Jordan River (flowing from the north) and the western catchments show high activity ratios between ∼1.5–1.7, and the eastern and southern catchments and flash floods (in the south-west, south and east) show lower ratios of 1.0–1.2. In Dead Sea water and precipitated minerals, 234U/238U is nearly always ∼1.45–1.55 during both glacials and interglacials. However, during the last interglacial MIS 5e insolation peak (∼127–122 ka) its value decreased to 1.2–1.3, and then to ∼1.0 towards its end (∼122–116 ka). During the insolation peak, the U-isotope data, combined with climate model runs forced with period orbital and greenhouse gas concentrations, indicate that rainfall associated with the African Summer Monsoon in the Dead Sea catchment accounted for ∼50% of the total annual rainfall, in stark contrast to present-day dry summers. The geochemical evidence indicates that following the insolation peak the region experienced an extremely dry period (although punctuated with wetter intervals), signifying expansion of the desert belt, similar to predicted effects of anthropogenic warming. This drying is partly supported by climate model runs forced with the appropriate changes in orbital parameters. The extreme drying during late MIS 5e between ∼122–116 ka reflected a major weakening of Mediterranean storm systems, resulting in a major decline of the Jordan River flow (indicated by the low 234U/238U ratios in the Dead Sea) and a relative increase in precipitation associated with the African Monsoon, shifting towards autumn. The Jordan River flow is estimated to be ∼10% of the present-day (pre-1964, prior to major diversion of the Jordan River and its sources for human use). Such changes, if they occur in the future, have serious implications for future water availability in the politically sensitive Middle East.

Holocene evolution of halite caves in the Cordillera de la Sal (Central Atacama, Chile) in different climate conditions

Geomorphological studies have been carried out in rapidly evolving salt caves related to small watersheds in the San Pedro de Atacama area, Chile. Radiocarbon ages of bones and wood from cave deposits, combined with the presence of large salt caves, geomorphological and sedimentological observations, and the results of micrometer measurements outside and in some of the caves, suggest a period of speleogenesis in the Cordillera de la Sal during the onset of the Holocene, during which the large halite cave systems developed, followed by an early Holocene hyperarid period. Most smaller caves (i.e. Lechuza del Campanario) most probably formed at the start of the wetter mid-Holocene period (5–4.4 ka), when precipitation was never intense enough to entrain large amounts of sediments, but enough to trigger cave development. A diamicton in Lechuza del Campanario Cave radiocarbon dated at ca. 4.4 ka shows that at least one high intensity rainfall event occurred in this recharge basin during the mid-Holocene wet interval. A wet period with lower intensity rainfall events followed between 4.0 and 2.5 ka, causing the 4.4 kyrs old diamicton in Lechuza del Campanario Cave to be entrenched, and the alluvial fan at the downstream end of Palacio del Sal Cave to be covered with windborne sediments dated by OSL at around 3.6 ka. At ca. 2 ka there was a high-intensity rainfall event documented by the age of a twig stuck in the ceiling of the Palacio del Sal Cave, followed by a period with lower intensity rain events until ca. 1.3 ka, when another intense flood produced a mudflow that deposited a second diamicton in Lechuza del Campanario Cave. From then on clustering of radiocarbon ages for wood and bone recovered from caves indicates increased rainfall intensity in the period ca. 0.9–0.5 ka, followed by no registered events until a minor flood at ca. 0.13 ka. The four-centuries long wetter time interval (0.9–0.5 ka), corresponding to the Medieval Climate Anomaly, has been an archeologically important period in the Atacama Desert (Tiwanaku culture).The observations and a detailed review of paleoclimate literature from this key area have allowed the development of a landscape evolution model related to changing climate conditions during the Late Holocene.

Landscape evolution and development of eolian-modified unconsolidated gravel surfaces and yardangs in the Hami Basin, China

Unconsolidated gravel surfaces, alluvial terraces, and yardangs are widespread in the Turpan-Hami topographical depression of NW China. These landscape features provide an opportunity to investigate the wind erosion history and landscape evolution of the Hami Basin. Along with anomalously high near-surface wind speeds, these features suggest that the Hami Basin was an important source of mineral dust during the Pleistocene. We present the results of an integrated geomorphologic, stratigraphic, and chronologic study on the genesis of unconsolidated gravel surfaces and yardang fields in the Hami Basin. Mobilization of silt- to sand-sized fractions from the surfaces of the Quaternary alluvial sediments occurred under generally dry conditions, leaving unconsolidated gravels to organize into inflationary-type desert pavement surfaces (Type-1) and deflationary-type desert pavements surfaces (Type-2). We propose that the Pleistocene alluvial terraces of the Hami Basin formed principally during wetter intervals, which are assumed to correspond with interglacial periods. The coarse-grained alluvium atop terraces protected underlying strata from wind erosion. The bedrock then began to be sculpted into yardangs after the removal of the coarse-grained armoring surfaces. With mostly sub-horizontal bedding, the geometries of yardangs was controlled by lithology, wind and water erosion. Based on UAV-3D models, optically-stimulated luminescence (OSL) and electron spin resonance (ESR) ages, we find an erosion rate of ~0.23 mm/yr since ~200 ka, This, combined with widespread uneroded strata armored by unconsolidated gravel surfaces, indicates that the Hami Basin had considerable dust emission potential in the past.

A lake sediment stable isotope record of late-middle to late Holocene hydroclimate variability in the western Guatemala highlands

Long-term perspectives on past hydroclimate variability provide context for evaluating the potential future impacts of changes in Northern Hemisphere temperatures and oceanic-atmospheric circulation on the timing and distribution of precipitation in Central America. Here we use the isotopic composition of fine-grained (< 63 μm), endogenic CaCO3 (δ18Ocalcite and δ13Ccalcite) from Lake Kail, located in the western highlands of Guatemala, to infer multi-decadal to multi-centennial-scale variability in the balance between precipitation and evaporation (P/E) over the last ∼6000 years. The sediment age model is based on 210Pb and 41 14C ages, with chronological uncertainty estimated using Bayesian methods. A model that couples lake hydrology and isotope mass balance is applied to characterize the isotopic responses of lake water and calcite to changes in drought-controlling climate variables such as precipitation, temperature and relative humidity. The δ18Ocalcite variations indicate intermediate-to-dry P/E conditions during the late-middle Holocene from ∼6000 to ∼4100 calyrBP. There was then a shift to drier conditions at the start of the late Holocene until ∼3050 calyrBP, followed by a trend to wetter conditions until ∼1500 calyrBP. The most recent ∼1500 years is characterized by high and relatively stable P/E, with particularly wet intervals from ∼1500 to 1170 calyrBP and from 470 to 260 calyrBP. Comparisons of the Lake Kail δ18Ocalcite record with Central American/circum-Caribbean proxy datasets spanning the late-middle and late Holocene indicate northern tropical hydroclimate changes were highly variable across space and time and were likely driven by regional oceanic-atmospheric responses with a secondary influence by insolation forcing.

Climate Change and the Pattern of the Hot Spots of War in Ancient China

Quantitative research on climate change and war hot spots throughout history is lacking. In this study, the spatial distribution and dynamic process of war hot spots under different climatic phases in imperial China (1–1911 CE) are revealed using Emerging Hot Spot Analysis (EHSA), based on the Global Moran’s Index for testing the degree of spatial autocorrelation or dependency. The results show that: (1) Battles were significantly clustered regardless of any climatic mode or war category. (2) Hot spots for all war were generally located in the Loess Plateau and the North China Plain during warm and wet periods, but in the Central Plain, the Jianghuai region, and the lower reaches of the Yangtze River/Yangtze River Delta during cold and dry conditions. (3) Hot spots for agri-nomadic conflict have similar patterns as those for all war, whereas rebellion hot spots expanded outward during warm and wet intervals yet contracted inward during cold and dry stages. These findings, by providing insightful evidence into the spatiotemporal patterns of war under the movements of climatic-ecological zones and geopolitical variations in ancient China, can be a starting point for future exploration of the long-term relationship between climate change and social security.

Geochemistry of last glacial lacustrine sediments in core region of the North American Monsoon, northwest Mexico: Source of biomass, hydrological balance and chemical weathering

Carbon isotopes (δ13Corg) in biomass, carbon (δ13Ccarb) and oxygen (δ18Ocarb) isotopes in authigenic calcite and estimations of chemical weathering of siliciclastic minerals (chemical index of alteration [CIA] and plagioclase index of alteration [PIA]) in lacustrine sediments of the ephemeral Babicora Basin, located within the core region of the North American Monsoon at northwest Mexico, helped to evaluate the source of biomass, variations in hydrological balance and sediment–water interactions during three different marine isotope stages (late MIS 4: ≥65.4–59.5 cal ka BP, MIS 3: ~59.5–28 cal ka BP and MIS 2: ~28–15 cal ka BP) of the last glacial. Comparisons of δ13Corg with previously published records of C/N, diatom abundance and fossil pollen indicate that biomass of the late MIS 4 was sourced from C3 aquatic and land plants. During the MIS 3, the C3 plants continued to contribute biomass and the influence of lacustrine algae, however, increased during its later part. The water column experienced more evaporation compared to two other MIS stages and sediment–water interactions varied between minimal to moderate (CIA: 49–68). Sediments were transported for longer distances during the wetter intervals and eroded from nearby lithologies during the drier intervals. Interactions between sediments and inflowing water (CIA: 59–65) remained homogeneous during the MIS 2. Source of the biomass, however, was variable. Lacustrine algae and C3 land plants contributed biomass during the early MIS 2 (~28–19 cal ka BP) and contributions from the C4 and CAM vegetation slightly increased towards the deglaciation (~19–15 cal ka BP).

Variable Late Holocene 14C Reservoir Ages in Lake Bosten, Northwestern China

Radiocarbon (14C) dating has been widely used in paleoclimate reconstruction. However, the reliability of the 14C age in lake sediments is sensitive to the 14C reservoir effect, especially for a lake in arid regions. In this study, we evaluated the 14C reservoir ages under different hydroclimatic conditions over the past ~2300 years in Lake Bosten, northwestern China, by comparing with different dating results and with multi-proxy indices of a vertical-down sediment core collected in this lake. The 14C reservoir ages during ~ 1800 to 650 BP (a dry interval) are estimated to be approximately 1170 years older than those during ~ 650 -100 BP and those during 2200-1800 BP (wet intervals). We proposed that variation in 14C exchange rate between the dissolved CO2 in lake water and CO2 in the atmosphere, as well as the changing proportion of organic matter in the lake and the catchment, could have contributed to the variable 14C reservoir ages in Lake Bosten. The result of this study suggests that the 14C reservoir effect may be larger in dry periods than in wet periods in the arid/semi-arid area, which should be considered when establishing age models of lake sediment cores from these regions.

Environmental changes on the west coast of the Gulf of Thailand during the 8.2 ka event

The expression of the abrupt cooling of the 8.2 ka event in the tropics is poorly understood. In this study, we applied lithostratigraphy, loss on ignition, grain size and pollen analyses to sedimentary sequences from Thale Noi, which is located in the west coast of the Gulf of Thailand, in order to reconstruct the regional environmental history during the 8.2 ka event. Our results, based on sediment. This characteristics and pollen records, indicated that the study area was likely to have flooded prior to c. 8300 cal. a BP. The fall in sea-level coincided with a high precipitation, inferred from grain size, between c. 8250 and 8100 cal a BP. The increased precipitation can be attributed to the strengthening of the northeast (winter) monsoonal rains in the western Gulf of Thailand. The timing of the sea-level fall and strengthen stronger winter monsoon is consistent with the abrupt decrease in the temperature records emanating from the northern hemisphere. Subsequently, we observed a sea-level rise from c. 8100 to 7950 cal a BP, which potentially continued till c. 7650 cal. a BP. The possibility of coastal erosion was detected by a hiatus in the sedimentary sequences dated between c. 7950 and 7650 cal a BP. Then, a standstill of the sea-level occurred from c. 7650 to 7100 cal a BP. This transgression is suggested to have caused coastal erosion that resulted in a hiatus in the sedimentation process between c. 7950 and 7650 cal a BP. Correspondingly, fluctuations in the monsoon strength resulted in an overall decreased precipitation from c. 8100 to 7450 cal a BP, although punctuated wet intervals were observed at c. 7950 and 7575 cal a BP.

High-resolution environmental magnetic study of a Holocene sedimentary record from Zaca Lake, California

Mineral magnetic studies of Holocene lake sediments recovered from Zaca Lake, in coastal southern California have yielded a 3000-year high-resolution record of terrestrial hydrologic variability and paleolimnology. Samples for magnetic analysis were obtained from an 8.73 m core recovered from the central region of Zaca Lake. Ages, constrained using a combined paleomagnetic and radiocarbon chronostratigraphy, yield sedimentation rates of 2–10 mm/yr with an average rate of 3 mm/yr over the 3000-year interval. Parameters reflecting decadal-scale variability in magnetic concentration (susceptibility, ARM, SIRM) and grain size (ARM/Chi) were measured every 2 cm. Additional rock magnetic tests on whole rock samples and magnetic separates indicate that magnetite and an iron sulfide are the chief magnetic carriers. Detailed analysis of the rock magnetic record reveals four major temporal intervals: 0–135 cm (~AD 2009–1757) the period of European settlement; 135–270 cm (~AD 1757–1275) the period of Native American, Chumash occupation of the watershed encompassing the bulk of the wet interval known as the little ice age (LIA); 270–600 cm (AD 1275–100) encompassing the drier Medieval Climate Anomaly (MCA); and 600–875 cm (2000–3000 ybp) an era dominated by a long interval of low magnetic mineral intensity and concentration that has been interpreted as a time of epic drought. Beginning with the cessation of drought conditions at Zaca Lake around ~2000 ybp, the magnetic proxy record of rock magnetic cyclicity and hydrologic variability shows a pronounced decadal to centennial-scale periodicity. We interpret these periodicities as oscillations between summer-dominated sedimentary sequences, characterized by whiting events with their resulting carbonate deposits, and winter dominated sedimentary sequences, typified by sulfide-rich clastics, reflecting lacustrine overturn and enhanced advection of detrital flux during winter storm events. The long-term variation of summer versus winter-dominated weather patterns reflects the climate variability of central California coast-ranges.

High‐resolution paleoclimatic records spanning the past 30 cal ka BP inferred from Qionghai Lake sediments in south‐west China: Insights from geochemical investigations and grain‐size characteristics

There have been few high‐resolution investigations covering the last glacial and Holocene from Sichuan, Yunnan, or south Asia area in the south‐west monsoon domain region. High‐resolution analyses of lake sediments retrieved from Qionghai Lake, Sichuan Province, were conducted using a combination of variables including total organic carbon (TOC) and total nitrogen (TN) concentration, low frequency magnetic susceptibility (χLF), particle size, carbonate content, and δ13C value of organic matter (δ13Corg), to provide a new perspective on the detailed paleoclimate history of south‐west China spanning the past 30 cal ka BP. Our findings suggest that organic matter in the Qionghai Lake was mainly derived from microorganisms, aquatic macrophytes, and terrestrial C3 plants. From 29.1 to 23.2 cal ka BP, low carbonate contents, high median grain size (Md values), and χLF values indicated a warm and wet climate. During the period 23.2 to 15.4 cal ka BP, low χLF, Md values, and high carbonate contents suggested a decrease in temperature and humidity. The cold and dry Last Glacial Maximum (LGM) and Heinrich 1 event (H1) were captured between 21.6 and 19.4 cal ka BP and 17.6 and 15.8 cal ka BP, respectively. During the interval 15.4 to 9.7 cal ka BP, low level of χLF values and Md values, together with relatively high carbonates, demonstrated that a dry climate prevailed in this period. Since 9.7 cal ka BP, the climate was warm and humid in the early to mid‐Holocene and shifted towards cold and dry condition in the late Holocene. A cold and dry period at 8.5 to 8.1 cal ka BP and warm and wet interval at 6.6 to 4.6 cal ka BP were identified, corresponding to the 8.2‐ka cold event and Holocene climatic optimum (HCO), respectively. These climate changes may reflect variations of the intensity of the south‐west summer monsoon, primarily in response to the Northern Hemisphere solar insolation on a millennial scale induced by orbital forcing. The position of the Intertropical Convergence Zone (ITCZ) may add a complementary effect on the evolution of the monsoon strength in the Holocene.

Subtropical hydroclimate during Termination V (∼430-422 ka): Annual records of extreme precipitation, drought, and interannual variability from Santa Barbara Basin

Hydroclimate extremes are expected to become more frequent and intense with anthropogenic climate forcing, but future El Niño-Southern Oscillation (ENSO) behavior is unclear. Understanding of extreme hydroclimate variability and magnitude prior to human-influences is limited by short instrumental records, however, continuous sedimentary archives of past hydroclimate variability can complement and extend these records. Laminated Santa Barbara Basin (SBB) sediment cores MV0508-33JPC, −21JPC, and −29JPC preserve annually-resolved, multi-centennial-scale precipitation records from Termination V (TV), the transition from glacial Marine Isotope Stage (MIS) 12 to interglacial MIS 11 at 424 ka. These records provide insights into the subtropical hydroclimate response to warming. Foraminiferal δ18O variations indicate rapid warming and/or salinity changes, and were used to verify the TV age assignment. A paleoprecipitation proxy was developed using the first principal component of scanning XRF elemental counts (PC1), which has high loadings for siliciclastic sediment-associated elements K, Ti, and Si. Sedimentary laminae couplets identified in PC1 were annually-tuned to investigate TV paleoprecipitation variability, as modern SBB laminae represent annual deposits. Extreme flooding and decadal-to-centennial droughts were identified, with magnitudes exceeding modern observations. ENSO-like (2–7 year) paleoprecipitation periodicities coincide with wetter intervals, but ENSO variability is reduced during droughts. A 1500-year arid interval may be related to poleward shifting of general atmospheric circulation as ice sheets melted, such that the subtropical dry zone intersected California. Southern California paleoprecipitation reconstructions from past warm climates provide insight into precipitation variability and magnitude on interannual timescales, and can reduce uncertainties in predictions of interannual climate, including ENSO.

Late Quaternary climate in southern China deduced from Sr–Nd isotopes of Huguangyan Maar sediments

The hydro-climatic conditions that prevailed during the last Glacial and early to mid-Holocene periods in South China are inferred from chemical compositions and Sr–Nd isotope ratios of sediments from lake Huguangyan Maar and its vicinity. The lake sediments are comprised of organic matter, volcanic materials and aeolian input from nearby granitoid-derived soils. Variations in 87Sr/86Sr ratios in the lake sediments indicate two modes of climate conditions: wet intervals during which the lake sediments are mainly derived from the volcanic-lake rim materials, expressed in low 87Sr/86Sr ratios, and dry intervals during which fine particles from the nearby granitic soils are windblown to the lake and supply local dust expressed in high 87Sr/86Sr ratios in the sediments. These wet and dry intervals generally correspond to regional climate records (e.g., speleothem δ18O profiles in southeast China) and correlate with global climate events (e.g., Heinrich events). While δ18O records of speleothems from southeast China caves are dominated by the precession signal, the Huguangyan Maar Sr record mainly correlates with obliquity. This most likely reflects masking of the precession signal due to regional climate variability, accentuating the obliquity signal. These local effects may also account for some of the differences that have been observed between the various East Asian monsoon records. More importantly, the masking of the precession signal reveals the direct influence of obliquity on the hydro-climate regime in South China.

A Field Guide to Finding Fossils on Mars.

The Martian surface is cold, dry, exposed to biologically harmful radiation and apparently barren today. Nevertheless, there is clear geological evidence for warmer, wetter intervals in the past that could have supported life at or near the surface. This evidence has motivated National Aeronautics and Space Administration and European Space Agency to prioritize the search for any remains or traces of organisms from early Mars in forthcoming missions. Informed by (1) stratigraphic, mineralogical and geochemical data collected by previous and current missions, (2) Earth's fossil record, and (3) experimental studies of organic decay and preservation, we here consider whether, how, and where fossils and isotopic biosignatures could have been preserved in the depositional environments and mineralizing media thought to have been present in habitable settings on early Mars. We conclude that Noachian‐Hesperian Fe‐bearing clay‐rich fluvio‐lacustrine siliciclastic deposits, especially where enriched in silica, currently represent the most promising and best understood astropaleontological targets. Siliceous sinters would also be an excellent target, but their presence on Mars awaits confirmation. More work is needed to improve our understanding of fossil preservation in the context of other environments specific to Mars, particularly within evaporative salts and pore/fracture‐filling subsurface minerals.

Late Holocene wet/dry intervals from Fuegian steppe at Laguna Carmen, southern Argentina, based on a multiproxy record

Late Holocene environmental conditions are reconstructed from a sedimentary core (LCTF2) retrieved from Laguna Carmen (53° 40′ 60″S, 68° 18′ 0″W, 29 m a.s.l.) in the Fuegian steppe, northern Tierra del Fuego, southern Argentina. The multiproxy study utilizes pollen/spores, algae, ostracods, palynofacies, Total Organic Carbon (TOC), carbonate content, and lithology. Findings show that grass communities developed over the landscape from 4200 to 1400 cal yr BP. After that, the plant communities fluctuated between grasses to scrubland vegetation. Changes in the lake level as indicated by the halophytes, algal content and ostracod associations, revealed alternation of wet and dry intervals. The palynofacies indicate environments close to the terrestrial source with a great input of terrigenous organic matter into the lake in agreement with the sedimentary environment. By comparison with other sites from Tierra del Fuego and southwestern Patagonia, the record of wind-carried Nothofagus pollen is consistent with variations in the Andean forest communities as a consequence of shifts in the latitudinal position and/or strength of the westerlies. The short-term wet/dry intervals seem to be associated with climate events of local occurrence. On the other hand, some wet intervals have been reported in lacustrine records from southern Patagonia. The last millennium showed high environmental variability. Humid conditions characterized the Medieval Climate Anomaly (MCA) event, while dryness conditions were related to the Little Ice Age (LIA) event. In particular this latter interval has been related to a northward migration of the westerlies from their present day focus.

May–June drought reconstruction over the past 821 years on the south-central Tibetan Plateau derived from tree-ring width series

Knowledge of drought variability and their possible mechanisms during the past hundred years is still limited in the mountainous region of south-central Tibetan Plateau (TP). In this study, a long-term tree-ring width chronology dating back to 1190 CE was combined using 328 increment cores from the Nagqu region. Based on the relationships between this tree-ring width chronology and climate data, we reconstructed May–June self-calibrated Palmer Drought Severity Index (scPDSI) for the past 821 years (1190–2010 CE). Additional comparisons with other available precipitation or drought reconstructions were conducted. We further investigated the influence of the South Asian summer monsoon (SASM) on the drought variability in our study region. Results indicated that our tree-ring width chronology contained stable drought signal in the early summer season (May–June). During the past 821 years, the longest dry and wet periods lasted for 116 and 90 years, respectively, based on a 21-year Fast Fourier transform filter. Specifically, longer than ten years’ dry periods prevailed during 1211–1245 CE, 1280–1358, 1421–1471, 1500–1571, 1580–1598, 1650–1691, 1782–1807 and 1867–1982; while wet intervals occurred in 1190–1210 CE, 1246–1279, 1359–1420, 1472–1499, 1599–1649, 1692–1781, 1808–1866 and 1983–2010. Generally consistent dry and wet intervals across the southern TP were found by comparisons with other available datasets during their common periods. Interestingly, we detected an unstable influence of the SASM on the May–June drought variability in our study region, at least for the past three and a half centuries. This study therefore gives a new perspective of drought variability as well as their relationships with the SASM over a long-term period on the south-central TP.

Lake levels in a discontinuous permafrost landscape: Late Holocene variations inferred from sediment oxygen isotopes, Yukon Flats, Alaska

ABSTRACT During recent decades, lake levels in the Yukon Flats region of interior Alaska have fluctuated dramatically. However, prior to recorded observations, no data are available to indicate if similar or more extreme variations occurred during past centuries and millennia. This study explores the history of Yukon Flats lake origins and lake levels for the past approximately 5,500 years from sediment analyses guided by previous work on permafrost extent, thermokarst, and modern isotope hydrology. Sediments dated by 210Pb and AMS radiocarbon indicate stable chronologies following initial lake initiation. Subsequent lithology is autochthonous, and oxygen isotope ratios of endogenic carbonate reflect lake level change at multiple time scales. Sediment results indicate high lake levels between approximately 4000 and 1850 cal yr BP, which is interpreted to reflect wetter-than-modern conditions. Lower lake levels with short-lived high stands during the past approximately 800 years reflect generally arid conditions with brief wet intervals similar to the region’s moisture regime today. The millennial trend is one of increasing aridity and corresponds closely with fire reconstructions and regional paleoclimatic trends. We conclude that high-magnitude lake-level fluctuations and decadal scale trends occurred before the observational period and are persistent hydroclimatic features of the Yukon Flats region.

A 9000-year flood history for Southern California: A revised stratigraphy of varved sediments in Santa Barbara Basin

The center of Santa Barbara Basin (SBB) preserves annual laminations through most of the Holocene providing an important locality for high-resolution late Quaternary paleoclimate and paleoceanography reconstructions. An accurate chronology is necessary for these sediment-based records and enables comparisons with more distal data time-series of similar quality. Here we present an improved high-resolution radiocarbon chronology for the last 9000yrs. based on 89 accelerator mass spectrometric (AMS) 14C dates of mixed planktonic foraminiferal carbonate from three sediment cores collected in SBB (MV0811-14JC, SPR0901-06KC and ODP Hole 893A). Accurate core-to-core correlation is demonstrated using prominent gray flood and olive turbidite layers identified in SBB and dated in multiple cores. Gray layer deposits were found more frequently in wetter intervals as determined by multi-proxy hydroclimate records in Southern California (e.g., lake deposits and tree rings). Mass accumulation rates (MARs) calculated for the SBB depocenter using the improved radiocarbon age model indicate high MARs are associated with these gray layers that have been associated with floods. Thus sediment accumulation in SBB is largely controlled by sediment delivery via river runoff following precipitation events although MARs may also be influenced by other factors that enhance erosion in river catchments. Folded sediment is observed within the basal core section of MV0811-14JC which stratigraphically correlates with 14C age reversals in ODP Hole 893A. We associate this sediment slump feature with the large Goleta submarine slide complex in Santa Barbara Channel and estimate the submarine slide event occurred at ~ 9000±200yrs. BP. Furthermore, small olive turbidite layers identified and dated in SBB can be temporally linked with large earthquakes along San Andreas Fault and can therefore potentially reconstruct earthquake history in Southern California.