Lake evolution and hydroclimate variation at Lake Qinghai (China) over the past 32 ka inferred from ostracods and their stable isotope composition

We used ostracod species assemblages and their δ¹⁸O values in a 32-m sediment core from Lake Qinghai, China, along with information from cores collected at other sites in the lake, to infer lake evolution and hydroclimate changes since the last glacial. Dominant ostracod species Ilyocypris bradyi and its low δ¹⁸O values showed that Lake Qinghai was small in size or even consisted of several playa lakes, and the 1F core site could have even been in a wetland setting, under cold and dry climate conditions before 15.0 ka. Presence of Limnocythere inopinata with low δ¹⁸O values, and absence of I. bradyi after 15.0 ka, indicate the lake area increased or that the playas merged. The decrease or disappearance of ostracods with high δ¹⁸O values showed that the lake shrunk under dry climate from 12.0 to 11.6 ka. After 11.6 ka, hydroclimate shifts inferred from ostracod species changes (Eucypris mareotica and L. inopinata) and their δ¹⁸O values were as follows: (1) 11.6–7.4 ka—larger, but still small lake area with greater moisture availability under primarily dry climate conditions, (2) 7.4 to 3.2 ka—increasing lake level under a warmer and wetter climate, and (3) 3.2 ka to present—stable, large, brackish lake. The low ratio of lake water volume to runoff, and close proximity of the core site to freshwater input from the river mouth would have resulted in relatively lower ostracod δ¹⁸O values when Lake Qinghai was small in area during the interval from 32.0 to 15.0 ka. Lower ostracod δ¹⁸O values during interstadials and throughout the entire Last Glacial Maximum and early deglacial (ca. 24.0–16.0 ka) were caused by a greater contribution of seasonal meltwater from ice or snow and low incoming precipitation δ¹⁸O values related to cold climate conditions in the region at that time.     

A 16-ka δ18O record of lacustrine sugar biomarkers from the High Himalaya reflects Indian Summer Monsoon variability

We investigated a late glacial–Holocene lacustrine sediment archive located at 4,050 m a.s.l. in the small carbonate-free catchment of Lake Panch Pokhari, Helambu Himal, Nepal. A δ¹⁸O sugar biomarker record was established by applying novel compound-specific δ¹⁸O analysis of plant sugar biomarkers (Zech and Glaser in Rapid Commun Mass Spectrom 23:3522–3532, 2009). This method overcomes analytical challenges such as extraction and purification faced by previous methods aimed at using δ¹⁸O of aquatic cellulose as a paleoclimate proxy. The δ¹⁸O results for sugar biomarkers arabinose, xylose and fucose agree well and reveal a pronounced trend towards lower δ¹⁸O values during the deglaciation and the onset of the Bølling/Allerød interstadial. By contrast, the period of the Younger Dryas is characterized by higher δ¹⁸O values. The early Holocene again reveals lower δ¹⁸O values. We suggest that our lacustrine δ¹⁸O record reflects coupled hydrological and thermal control. It is strongly related to changes in the oxygen isotopic composition of paleo-precipitation and resembles the δ¹⁸O records of Asian speleothems. With respect to the ‘amount effect,’ the record is interpreted as reflecting the Indian Summer Monsoon intensity. The precipitation signal is, however, amplified in our record by evaporative ¹⁸O enrichment that is controlled by the ratio of precipitation to evaporation. We suggest that our δ¹⁸O record reflects the variability of the Indian Summer Monsoon, which was strong during the Bølling/Allerød interstadial and early Holocene, but weak during the Younger Dryas stadial. This interpretation is corroborated by a pollen-based index for Lake Panch Pokhari that estimated the strength of the Indian Summer Monsoon versus the strength of the Westerlies. Millennial-scale synchronicity with the Greenland δ¹⁸O temperature records highlights the previously suggested strong teleconnections between the Asian Monsoon system and North Atlantic climate variability.     

Aquatic macrophyte dynamics in Lake Karakul (Eastern Pamir) over the last 29 cal ka revealed by sedimentary ancient DNA and geochemical analyses of macrofossil remains

Due to methodological challenges there are only a few studies that focus on macrophyte dynamics in large lakes despite their notable role in a lake’s ecosystem functioning. This study investigates composition and productivity changes of the submerged vegetation of Lake Karakul, Pamir Mountains (Tajikistan), using sedimentary ancient DNA metabarcoding and elemental (C/N) and isotopic (δ¹³C, δ¹⁵N) measurements of Stuckenia cf. pamirica (Baagøe) Z. Kaplan (Potamogetonaceae) leaf remains. No Stuckenia cf. pamirica leaf remains were found for 28.7–26.1 cal ka BP, when both Potamogetonaceae and Chara (L.) DNA sequences were recorded, suggesting sparse submerged vegetation at the coring site. This agrees with the inference of a deep lake reached using geochemical proxies. From 26.1 to 17.5 cal ka BP a few macrophyte remains and high numbers of Potamogetonaceae sequences were recovered: lake level was probably low, as suggested by other studies on the lake. Another phase of increased numbers of Chara sequences and the absence of Stuckenia cf. pamirica leaf remains was found between 17.5 and 12.2 cal ka BP, which coincides with a lake-level transgression at Lake Karakul as indicated by paleo-shoreline investigations. Analyses of macrophyte remains reveal intermediate paleo-productivity from 6.9 cal ka BP and high paleo-productivity from 2.2 cal ka BP onwards. From comparisons with other studies, we suggest that lake-level changes are the main driver for the submerged vegetation composition and productivity at the coring site in Lake Karakul and underline our conclusions by depicting the present-day distribution of Stuckenia cf. pamirica and Chara within the lake.     

Late Holocene hydrologic changes in northern New Zealand inferred from stable isotope values of aquatic cellulose in sediments from Lake Pupuke

Isotopic records of aquatic cellulose are becoming increasingly important for palaeohydrological reconstructions, but widespread application of this climate proxy is hampered by minerogenic contamination that affects oxygen isotope measures in cellulose. Few records of isotopes in aquatic cellulose are available from palaeoclimate archives in the Southern Hemisphere. In this study, we used a new bulk cellulose extraction method and determined the oxygen (δ¹⁸O) and carbon (δ¹³C) isotope values in cellulose from a Holocene lake sediment core segment (7.2–1.1 cal ka BP) from Lake Pupuke, Auckland, New Zealand. Isotope values from modern, potential sources of sedimentary cellulose revealed the aquatic origin of the cellulose extracted from the core, and hence enabled inference of past lake water δ¹⁸O values from the δ¹⁸O of measured cellulose in the core. A shift to a more positive water balance in the lake was identified around 2.8 cal ka BP by a decrease in inferred lake water δ¹⁸O values. At that time, greater epilimnetic primary productivity is indicated by the higher δ¹³C values of sedimentary cellulose. Greater divergence between the δ¹³C values of cellulose and bulk organic matter suggests stronger stratification of the lake, likely caused by greater freshwater input. We discuss a possible link to a solar minimum that occurred at that time.     

Climatic and limnological changes at Lake Karakul (Tajikistan) during the last ~29 cal ka

We present results of analyses on a sediment core from Lake Karakul, located in the eastern Pamir Mountains, Tajikistan. The core spans the last ~29 cal ka. We investigated and assessed processes internal and external to the lake to infer changes in past moisture availability. Among the variables used to infer lake-external processes, high values of grain-size end-member (EM) 3 (wide grain-size distribution that reflects fluvial input) and high Sr/Rb and Zr/Rb ratios (coinciding with coarse grain sizes), are indicative of moister conditions. High values in EM1, EM2 (peaks of small grain sizes that reflect long-distance dust transport or fine, glacially derived clastic input) and TiO₂ (terrigenous input) are thought to reflect greater influence of dry air masses, most likely of Westerly origin. High input of dust from distant sources, beginning before the Last Glacial Maximum (LGM) and continuing to the late glacial, reflects the influence of dry Westerlies, whereas peaks in fluvial input suggest increased moisture availability. The early to early-middle Holocene is characterised by coarse mean grain sizes, indicating constant, high fluvial input and moister conditions in the region. A steady increase in terrigenous dust and a decrease in fluvial input from 6.6 cal ka BP onwards points to the Westerlies as the predominant atmospheric circulation through to present, and marks a return to drier and even arid conditions in the area. Proxies for productivity (TOC, TOC/TN, TOC _Br ), redox potential (Fe/Mn) and changes in the endogenic carbonate precipitation (TIC, δ¹⁸O _Carb ) indicate changes within the lake. Low productivity characterised the lake from the late Pleistocene until 6.6 cal ka BP, and increased rapidly afterwards. Lake level remained low until the LGM, but water depth increased to a maximum during the late glacial and remained high into the early Holocene. Subsequently, the water level decreased to its present stage. Today the lake system is mainly climatically controlled, but the depositional regime is also driven by internal limnogeological processes.     

Basin evolution,diagenesis and uranium mineralization in the Paleoproterozic Thelon Basin,Nunavut, Canada

The Paleoproterozoic (Statherian) Thelon Basin is located in the Churchill Province of the Canadian Shield, formed following the Trans‐Hudson Orogeny. Basin formation followed an interval of felsic volcanism and weathering of underlying bedrock. The diagenetic evolution of the Thelon lasted about one billion years and was punctuated by fluid movement influenced by tectonic events. Early quartz cements formed in well‐sorted, quartz‐rich facies during diagenetic stage 1; fluids in which these overgrowths formed had δ¹⁸O values near 0‰ (Vienna Standard Mean Ocean Water). Uranium‐rich apatite cement (P1) also formed during diagenetic stage 1 indicating that oxygenated, uranium‐bearing pore water was present in the basin early in its diagenetic history. Syntaxial quartz cement (Q1) formed in water with δ¹⁸O from ?4 to ?0.8‰ in diagenetic stage 2. Diagenetic stage 3 occurred when the Thelon Formation was at ca. 5 km depth, and was marked by extensive illitization, alteration of detrital grains, and uranium mineralization. Basin‐wide, illite crystallized at ～200 °C by fluids with δ¹⁸O values of 5–9‰ and δD values of ?60 to ?31‰, consistent with evolved basinal brines. Tectonism caused by the accretion of Nena at ca. 1600 Ma may have provided the mechanism for brine movement during deep burial. Diagenetic stage 4 is associated with fracturing and emplacement of mafic dikes at ca. 1300 Ma, quartz cement (Q3) in fractures and vugs, further illitization, and recrystallization of uraninite (U2). Q3 cements have fluid inclusions that suggest variable salinities, δ¹⁸O values of 1.5–9‰, and δD values of ?97 to ?83‰ for stage 4 brines. K‐feldspar and Mg‐chlorite formed during diagenetic stage 5 at ca. 1000 Ma in upper stratigraphic sequences, and in the west. These phases precipitated from low‐temperature, isotopically distinct fluids. Their distribution indicates that the basin hydrostratigraphy remained partitioned for >600 Ma.     

A 16-ka lake-level record inferred from macrofossils in a sediment core from Genggahai Lake, northeastern Qinghai–Tibetan Plateau (China)

Genggahai Lake, a small, shallow water body on the northeastern Qinghai–Tibetan Plateau, is presently occupied by dense aquatic vegetation. The distribution of macrophytes is strongly associated with water depth. Macrofossils from a 7.82-m sediment core indicate that the dominant taxa in past aquatic plant communities were almost the same as those that dominate the lake today. In combination with sediment geochemical variables TOC, TN, and δ¹³C_bulk-org, macrofossil assemblages (aquatic plant remains, stem encrustations, and mollusc shells) were employed to reconstruct lake-level fluctuations over the past 16 ka. The lake formed or refilled at 15.3 cal ka and sustained a low level until 11.4 cal ka. From 11.4 to 6.3 cal ka, a remarkably high level was punctuated by a period of relatively low level between 9.2 and 7.4 cal ka. Stepwise drops in lake level occurred from 6.3 to 5.5 cal ka. Water level was characterized by more frequent fluctuations between 5.5 and 4.1 cal ka. Low level persisted from 4.1 to 2.1 cal ka and from 1.6 to 0.3 cal ka. Short-lived episodes of high water level were centered around 1.8 cal ka and since 0.3 cal ka. The lake-level history of Genggahai Lake appears to be largely consistent with the Asian monsoon records from nearby and distant locations. Our preliminary results suggest that monsoon variability on centennial to millennial timescales was superimposed on the long-term pattern of the Asian monsoon, modulated by orbitally induced summer insolation, and that the Asian monsoon may play an important role in water balance in marginal regions.     

ENSO and solar activity signals from oxygen isotopes in diatom silica during late glacial-Holocene transition in Central Andes (18°S)

The late glacial-Holocene transition from the Lago Chungará sedimentary record in northern Chilean Altiplano (18°S) is made up of laminated sediments composed of light-white and dark-green pluriannual couplets of diatomaceous ooze. Light-white sediment laminae accumulated during short-term extraordinary diatom blooms whereas dark-green sediment laminae represent the baseline limnological conditions during several years of deposition. Diatom oxygen isotope ratios (δ¹⁸O_diatom) from 40 consecutive dark-green laminae, ranging from 11,990 to 11,450 cal year BP, show that a series of decadal-to-centennial dry–wet oscillations occurred. Dry periods are marked by relatively high isotope values whereas wet episodes are indicated by lower values. This interpretation agrees with the reconstructions of terrigenous inputs and regional effective moisture availability carried out in the lake but there is a systematic temporal disagreement between them owing to the non-linear response of the lacustrine ecosystem to environmental forcings. Furthermore, the δ¹⁸O_diatom record tracks effective moisture changes at a centennial scale. Three major phases have been established (11,990–11,800, 11,800–11,550, and 11,550–11,450 cal year BP). Each phase is defined by an increasing isotope trend followed by a sudden depletion. In addition, several wet and dry events at a decadal scale are superimposed onto these major trends. Spectral analyses of the δ¹⁸O_diatom values suggest that cycles and events could have been triggered by both El Niño-Southern Oscillation (ENSO) and solar activity. Significant ENSO frequencies of 7–9 years and 15–17 years, and periodicities of the solar activity cycles such as 11 years (Schwabe), 23 years (Hale) and 35 years (Brückner) have been recognised in the oxygen isotope time series. Time–frequency analysis shows that although solar and ENSO forcing were present at the onset of the Holocene, they were more intense during the late glacial period. The early Holocene might have been mainly governed by La Niña-like conditions that correspond to wet conditions over the Andean Altiplano.     

Defining drivers of nitrogen stable isotopes (δ15N) of surface sediments in temperate lakes

The nitrogen stable isotopic signature (δ¹⁵N) of sediment is a powerful tool to understand eutrophication history, but its interpretation remains a challenge. In a large-scale comparative approach, we identified the most important drivers influencing surface sediments δ¹⁵N of 65 lakes from two regions of Canada using proxies that reflect watershed nitrogen (N) sources, internal lake microbial cycling and productivity. Across regions, we found that water column total nitrogen (TN),  %N in the sediments and lake morphometric variables were the best predictors of sedimentary δ¹⁵N, explaining 66 % of its variation. Significant relationships were also found between sediment δ¹⁵N and human-derived N load ( \( R_{{{\text{adj}} .}}^{2} \)  = 0.23, p < 0.001), the latter being a strong predictor of TN ( \( R_{{{\text{adj}} .}}^{2} \)  = 0.68, p < 0.001). Despite a relatively strong overall relationship, variation partitioning revealed an interesting difference in the dominant variable that influenced regional δ¹⁵N. Alberta lake sedimentary δ¹⁵N signature was dominated by human derived N load. In contrast, internal processing appeared to be more important in Quebec lakes, where sediment δ¹⁵N was best explained by  %N in the sediments and lake volume. Overall, our findings support the use of δ¹⁵N in paleolimnological investigations to reconstruct changing N sources to lakes but also highlight that regions may have distinctive drivers. Interpretations of sediment δ¹⁵N are likely to be strongest when multiple lines of evidence are employed and when placed in a regional context.     

Delivery and deposition of organic matter in surface sediments of Lagoa do Caçó (Brazil)

Elemental and isotopic compositions of organic matter in surficial sediments from five transects across Lagoa do Caçó (Brazil) were analyzed to identify the depth-related processes that affect the production and deposition of sedimentary organic matter in this shallow tropical lake. Each of four transverse transects began at a margin dominated by aquatic macrophytes (Eleocharis), crossed the central deep part of the lake, and terminated in the opposite, macrophyte-dominated margin. In each transect, TOC concentrations, C/N ratios, and δ¹³C values decreased between 0 and 4 m, whereas δ¹⁵N values increased. The variables remained stable in sediment from 4 m water depth to the center of the lake at 10 m. The depth-related patterns reflect differences in both the delivery and the deposition of organic matter in the lake. Organic matter is produced in abundance in the marginal area by emersed and submerged macrophyte vegetation that diminishes with depth and disappears at 4 meters. After the disappearance of macrophytes, organic matter is produced at low rates principally by open-lake phytoplankton. Drawdown of dissolved oxygen is high in the lake margins, but it is low in the oligotrophic open waters of the lake. Preservation of organic matter is consequently better in sediments of the lake margins than in deep waters. The depth-related pattern of organic matter delivery and deposition in the sediments of Lagoa do Caçó, in which water levels are sensitive to groundwater fluctuations, shows that the elemental and isotopic compositions of sediment organic matter can provide a record of changes in the paleohydrology of this and other similar shallow lake systems.     

Palaeoclimate inferred from δ18O and palaeobotanical indicators in freshwater tufa of Lake Äntu Sinijärv,Estonia

We investigated a 3.75-m-long lacustrine sediment record from Lake Äntu Sinijärv, northern Estonia, which has a modeled basal age >12,800 cal yr BP. Our multi-proxy approach focused on the stable oxygen isotope composition (δ¹⁸O) of freshwater tufa. Our new palaeoclimate information for the Eastern Baltic region, based on high-resolution δ¹⁸O data (219 samples), is supported by pollen and plant macrofossil data. Radiocarbon dates were used to develop a core chronology and estimate sedimentation rates. Freshwater tufa precipitation started ca. 10,700 cal yr BP, ca. 2,000 years later than suggested by previous studies on the same lake. Younger Dryas cooling is documented clearly in Lake Äntu Sinijärv sediments by abrupt appearance of diagnostic pollen (Betula nana, Dryas octopetala), highest mineral matter content in sediments (up to 90 %) and low values of δ¹⁸O (less than ?12 ‰). Globally recognized 9.3- and 8.2-ka cold events are weakly defined by negative shifts in δ¹⁸O values, to ?11.3 and ?11.7 ‰, respectively, and low concentrations of herb pollen and charcoal particles. The Holocene thermal maximum (HTM) is palaeobotanically well documented by the first appearance and establishment of nemoral thermophilous taxa and presence of water lilies requiring warm conditions. Isotope values show an increasing trend during the HTM, from ?11.5 to ?10.5 ‰. Relatively stable environmental conditions, represented by only a small-scale increase in δ¹⁸O (up to 1 ‰) and high pollen concentrations between 5,000 and 3,000 cal yr BP, were followed by a decrease in δ¹⁸O, reaching the most negative value (?12.7 ‰) recorded in the freshwater tufa ca. 900 cal yr BP.     

Evidence of late Holocene climate variability in the western Nanling Mountains,South China

A 95-cm-thick peat sequence obtained from Daping Swamp in the western Nanling Mountains provides evidence for climate variability in the past ~3,000 year. Multi-proxy records (including organic carbon isotopes, humification degree, organic matter content, and dry bulk density) revealed three intensified Asian summer monsoon (ASM) intervals (i.e.~2900–2700, 2500–1700 and 1000–600 cal. yr BP) and three weakened ASM intervals (i.e.~2800–2500, 1700–1000 and 600–200 cal. yr BP). Our δ¹³C record shows a possible correlation with the sunspot number and residual atmospheric ¹⁴C records on multi-centennial scale, especially for the period between 2960 and 2200 cal. yr BP. A spectral analysis of δ¹³C record reveals three significant cycles (i.e., 396, 110 and 102 yr) and all these cycles could be related to solar activity, suggesting that solar output may have influenced the late Holocene climate variability in the study region.     

Post-glacial climate change and its effect on a shallow dimictic lake in Nova Scotia,Canada

A high-resolution, multi-proxy lake sediment record was used to establish the timing of Holocene environmental change in Canoran Lake, southwest Nova Scotia, Canada. Proxies include %C, δ¹⁵N, δ¹³C, HI, magnetic susceptibility, and pollen. Canoran Lake is a small, shallow (11 m) lake with two ephemeral inlets and an outlet. The site was deglaciated at ca. 15,300 cal (calibrated) year BP and elevated %C values indicate the establishment of a productive aquatic environment that is consistent with Allerød warming. The Allerød was interrupted by rapid air temperature cooling during the Younger Dryas (ca. 12,900–11,600 cal year BP). The Early Hypsithermal (ca. 11,600–8,500 cal year BP) was relatively warm and wet. A slight increase in clastic input occurred between 9,100 and 8,500 cal year BP but δ¹⁵N, δ¹³C, and HI values imply that the lithostratigraphic response may not be indicative of climate-induced change. The strong proxy response between 8,500 and 8,000 cal year BP was likely due to cooling and drying coincident with the 8.2 k year event. The climate was relatively warm and dry during the Late Hypsithermal (ca. 8,000–3,500 cal year BP). None of the proxies’ exhibit notable change during the 5,500 cal year BP hemlock decline, indicating that ecological change was likely due to a pathogen attack. Post-Hypsithermal (modern) climate was characterized by an increase in precipitation and a decrease in air temperatures from ca. 3,500 to 700 cal year BP (top of core).     

A 100-year record of changes in organic matter characteristics and productivity in Lake Bhimtal in the Kumaon Himalaya, NW India

Sediment variables total organic carbon (TOC), total nitrogen (TN), total sulfur (TS), as well as their accumulation rates and atomic ratios (C/N and C/S), were studied along with stable isotopes (δ¹³C, δ¹⁵N, and δ³⁴S), and specific biomarkers (n-alkanes and pigments) in a 35-cm-long sediment core from Lake Bhimtal, NW India. The average sedimentation rate is 3.6 mm year^?1, and the core represents a provisional record of ~100 years of sedimentation history. Bulk elemental records and their ratios indicate that sediment organic matter (OM) is derived primarily from algae. In-lake productivity increased sharply over the last two decades, consistent with paleoproductivity reconstructions from other lakes in the area. An up-core decrease in δ¹³C values, despite other evidence for an increase in lake productivity, implies that multiple biogeochemical processes (e.g. external input of sewage or uptake of isotopically depleted CO₂ as a result of fossil fuel burning) influence the C isotope record in the lake. The δ¹⁵N values (?0.2 to ?3.9 ‰) reflect the presence of N-fixing cyanobacteria, and an increase in lake productivity. The δ³⁴S profile shows enrichment of up to 5.6 ‰, and suggests that sulfate reduction occurred in these anoxic sediments. Increases in total n-alkane concentrations and their specific ratios, such as the Carbon Preference Index (CPI) and Terrestrial Aquatic Ratio (TAR), imply in-lake algal production. Likewise, pigments indicate an up-core increase in total concentration and dominance of cyanobacteria over other phytoplankton. Geochemical trends indicate a recent increase in the lake’s trophic state as a result of human-induced changes in the catchment. The study highlights the vulnerability of mountain lakes in the Himalayan region to both natural and anthropogenic processes, and the difficulties associated with reversing trophic state and ecological changes.     

The sedimentary record of the Issyk Kul basin,Kyrgyzstan: climatic and tectonic inferences

A broad array of new provenance and stable isotope data are presented from two magnetostratigraphically dated sections in the south‐eastern Issyk Kul basin of the Central Kyrgyz Tien Shan. The results presented here are discussed and interpreted for two plausible magnetostratigraphic age models. A combination of zircon U‐Pb provenance, paleocurrent and conglomerate clast count analyses is used to determine sediment provenance. This analysis reveals that the first coarse‐grained, syn‐tectonic sediments (Dzhety Oguz formation) were sourced from the nearby Terskey Range, supporting previous thermochronology‐based estimates of a ca. 25–20 Ma onset of deformation in the range. Climate variations are inferred using carbonate stable isotope (δ¹⁸O and δ¹³C) data from 53 samples collected in the two sections and are compared with the oxygen isotope compositions of modern water from 128 samples. Two key features are identified in the stable isotope data set derived from the sediments: (1) isotope values, in particular δ¹³C, decrease between ca. 26.0 and 23.6 or 25.6 and 21.0 Ma, and (2) the scatter of δ¹⁸O values increased significantly after ca. 22.6 or 16.9 Ma. The first feature is interpreted to reflect progressively wetter conditions. Because this feature slightly post‐dates the onset of deformation in the Terskey Range, we suggest that it has been caused by orographically enhanced precipitation, implying that surface uplift accompanied late Cenozoic deformation and rock uplift in the Terskey Range. The increased scatter could reflect variable moisture source or availability caused by global climate change following the onset of Miocene glaciations at ca. 22.6 Ma, or enhanced evaporation during the Mid‐Miocene climatic optimum at ca. 17–15 Ma.     

Holocene climate and environmental changes inferred from sediment characteristics and diatom assemblages in a core from Hwajinpo Lagoon,Korea

Hwajinpo is the largest lagoon in Korea and is located along the east coast of the country. It possesses Holocene sediments that provide an important record of past climate change. We studied the evolution of Hwajinpo Lagoon using grain size data and diatom assemblages in an 11.0-m core (HJ02), which was obtained at the mouth of a small river that drains into the lagoon. Core chronology was established with accelerator mass spectrometry ¹⁴C dates and optically stimulated luminescence dates. Diatom assemblages and grain size analysis revealed that estuarine conditions in the inner lagoon area transitioned to an open embayment ca. 8 ka as a result of sea-level rise. Around 7.8 ka, the open bay became a semi-closed bay as a consequence of development of a sand barrier. After the bay was semi-closed, marine water inflow was increasingly restricted as the sand barrier developed, and the semi-closed bay became a completely enclosed, low-salinity, brackish lagoon around 6 ka. There was an erosional hiatus between 5.5 and 1.7 ka (7.0 m depth), likely caused by river flooding and a switch in the location of drainage along the delta. The lagoon became oligohaline around 1.6 ka, likely because of increasing precipitation associated with an intensified Asian summer monsoon. This increase in precipitation resulted in expansion of the sand bar by sediment inflow, driven by agricultural development in the area. About 1000 years ago, the diatom assemblage was similar to the modern assemblage, suggesting the lagoon’s current geomorphic conditions had been established.     

A 15.4-ka paleoclimate record inferred from δ<Superscript>13</Superscript>C and δ<Superscript>15</Superscript>N of organic matter in sediments from the sub-alpine Daping Swamp,western Nanling Mountains,South China

We inferred past climate conditions from the δ¹³C and δ¹⁵N of organic matter (OM) in a sediment core (DP-2011-02) from the sub-alpine Daping Swamp, in the western Nanling Mountains, South China. In the study region, a 1000-m increase in altitude results in a ~0.75‰ decrease in δ¹³C and a ~2.2‰ increase in δ¹⁵N. Organic carbon stable isotope (δ¹³C) values of the dominant modern vegetation species, surface soils, and the core samples taken in the swamp exhibit a strong terrestrial C3 plant signature. Comprehensive analysis of the core indicates both terrestrial and aquatic sources contribute to the OM in sediment. Temperature and precipitation are most likely the critical factors that influence δ¹³C: warm and wet conditions favor lower δ¹³C, whereas a dry and cool climate leads to higher δ¹³C values. Higher δ¹⁵N values may result from greater water depth and increased primary productivity, promoted by large inputs of dissolved inorganic nitrogen, induced by high surface runoff. Lower δ¹⁵N values are associated with lower lake stage and reduced productivity, under drier conditions. Therefore, stratigraphic shifts in these stable isotopes were used to infer past regional climate. Measures of δ¹³C and δ¹⁵N in deglacial deposits, in combination with total organic carbon (TOC) and nitrogen (TN) concentrations, the TOC/TN ratio, coarse silt and sand fractions, dry bulk density and low-frequency mass magnetic susceptibility, reveal two dry and cold events at 15,400–14,500 and 13,000–11,000 cal a BP, which correspond to Heinrich event 1 and the Younger Dryas, respectively. A pronounced warm and wet period that occurred between those dry episodes, from 14,500 to 13,000 cal a BP, corresponds to the Bølling–Allerød. The δ¹³C and δ¹⁵N data, however, do not reflect a warm and wet early Holocene. The Holocene optimum occurred between ~8000 and 6000 cal a BP, which is different from inferences from the nearby Dongge cave stalagmite δ¹⁸O record, but consistent with our previous results. This study contributes to our understanding of climate-related influences on δ¹³C and δ¹⁵N in OM of lake sediments in South China.     

A ∼33,000 year record of environmental change from Arolik Lake, Ahklun Mountains, Alaska, USA

A continuous record of lacustrine sedimentation capturing the entire full-glacial period was obtained from Arolik Lake in the Ahklun Mountains, southwestern Alaska. Fluctuations in magnetic susceptibility (MS), grain size, organic-matter (OM) content, C/N ratios, ¹³C, and biogenic silica (BSi) record marked environmental changes within the lake and its watershed during the last 33 cal ka. Age control is provided by 31 ¹⁴C ages on plant macrofossils in four cores between 5.2 and 8.6 m long. Major stratigraphic units are traceable throughout the lake subbottom in acoustical profiles, and provisional ages are derived for six prominent tephra beds, which are correlated among the cores. During the interstadial interval between 33 and 30 cal ka, OM and BSi contents are relatively high with values similar to those of the Pleistocene–Holocene transition, suggesting a similar level of aquatic productivity. During the glacial interval that followed (30–15 cal ka), OM and BSi decrease in parallel with declining summer insolation. OM and BSi values remain relatively uniform compared with the higher variability before and after this interval, and they show no major shifts that might correlate with climate fluctuations evidenced by the local moraine record, nor with other global climate changes. The glacial interval includes a clay-rich unit with a depauperate diatom assemblage that records the meltwater spillover of an ice-dammed lake. The meltwater pulse, and therefore the maximum extent of ice attained by a major outlet glacier of the Ahklun Mountain ice cap, lasted from 24 to 22 cal ka. The Pleistocene–Holocene transition (15–11 cal ka) exhibits the most prominent shifts in OM and BSi, but rapid and dramatic fluctuations in OM and BSi continue throughout the Holocene, indicating pronounced paleoenvrionmental changes.     

Response of Sandy Lake in Schirmacher Oasis,East Antarctica to the glacial-interglacial climate shift

Freshwater lakes in Antarctica fluctuate from ice-free state (during austral summer) to ice-cover state (during austral winter). Hence the lakes respond instantly to the seasonal climate of the region. The Antarctic seasons respond sharply to the glacial and interglacial climates and these signatures are archived in the lake sediments. A sediment core from Sandy Lake, a periglacial lake located in Schirmacher Oasis of East Antarctica records distinct changes in grain-size, C, N, C/N ratios (atomic), δ¹³C_OM and δ¹⁵N_OM contents during the last 36 ky. The contents of the sedimentary organic matter (OM) proxies (C_org ~ 0.3 ± 0.2%, C/N ratios ~9 ± 5 and δ¹³C_OM ~?18 ± 6‰) indicate that the OM in this lake sediment is a product of mixing of terrestrial and lacustrine biomass. Distinctly lower contents of C_org (~0.2%) and sand (~50%), low C/N ratios (~8) and depleted δ¹³C_OM (~?20‰) during the Last Glacial Maximum (LGM: 32–17 ky BP based on Vostok Temperatures) suggest greater internal (autochthonous) provenance of organic matter and limited terrestrial (allochthonous) inputs probably due to long and intense winters in the Antarctic. Such intense winters might have resulted the lake surface to be ice-covered for most part of the year when the temperatures remained consistently colder than the Holocene temperatures. The denitrification within the lake evident by enriched δ¹⁵N_OM (>10‰) during Antarctic LGM might have resulted from oxygen-limitation within the lake environment caused by insulated lake surface. The gradual increases in δ¹³C_OM, C/N and sand content starting at ~11 ky BP and attaining high values (~?11‰, ~10 and ~80% respectively) at ~6 ky BP together suggest a subtle change in the balance of sources of organic matter between algal and macrophyte/bryophyte nearly 8–9 ky later to the beginning of the deglaciation. Thus the seasonal opening-up of the Sandy Lake similar to the modern pattern started with the establishment of the optimum temperature conditions (i.e., 0 °C anomaly) in the Antarctic, prior to which the lake environment might have remained mostly insulated or closed.     

Last deglaciation and Holocene environmental change at high altitude in the Pyrenees: the geochemical and paleomagnetic record from Marboré Lake (N Spain)

Sedimentological, geochemical and magnetic data in a ~ 7-m sequence from Marboré Lake (2612 m asl, central Pyrenees) provide information about environmental variability since the last glacier retreat (14.6 cal ka BP) in high-altitude Pyrenean environments. The sediment sequence is composed of millimeter- to centimeter-thick rhythmites made of finer greyish laminae and coarser-grain, carbonate-bearing laminae arranged in varied patterns throughout the sequence. Finer laminae are interpreted as deposition during periods of predominantly ice-covered conditions, whereas coarser, carbonate-bearing sediments reflect periods of higher runoff. The age model, based on 13 ¹⁴C dates and a reservoir effect assessed with ²¹⁰Pb and ¹³⁷Cs, is coherent with known synchronous vegetation changes across the Pyrenees. Warmer intervals such as GI-1 (14.6–12.8 cal ka BP, Bølling/Allerød period), 10.4–8.2 cal ka BP in the Early Holocene, 7.5–5.2 cal ka BP in the Mid Holocene and the Medieval Climate Anomaly (AD 900–1300), are characterized by peaks in productivity and higher carbonate preservation. Deposition during colder periods such as GS-1 (12.8–11.7 cal ka BP), the Neoglacial (ca. 5.2–3.5 cal ka BP) and the Little Ice Age (last 400 years) show an increase in finer laminae. The presence of magnetite throughout the whole section suggests that Marboré Lake maintained predominantly oxic conditions since its formation. Changes in magnetic properties and the increase in magnetite from 3.5 cal ka BP to present, however, indicate a more oxic environment at the lake bottom during the last few millennia. The occurrence of Pb concentration peaks in sediments of Roman and modern age demonstrates the global distribution of heavy metal deposition, even into high-mountain lakes.