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Climatic and limnological changes at Lake Karakul (Tajikistan) during the last similar to 29 cal ka
(2017)
We present results of analyses on a sediment core from Lake Karakul, located in the eastern Pamir Mountains, Tajikistan. The core spans the last similar to 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, TOCBr), redox potential (Fe/Mn) and changes in the endogenic carbonate precipitation (TIC, delta(18) OCarb) 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.
Stable carbon isotopes of sediment organic matter (delta C-13(OM)) are widely applied in paleoenvironmental studies. Interpretations of delta C-13(OM), however, remain challenging and factors that influence delta C-13(OM) may not apply across all lakes. Common explanations for stratigraphic shifts in delta C-13(OM) include changes in lake productivity or changes in inputs of allochthonous OM. We investigated the influence of different oxygen conditions (oxic versus anoxic) on the delta C-13(OM) values in the sediments of Lake Tiefer See. We analysed (1) a long sediment core from the deepest part of the lake, (2) two short, sediment-water interface cores from shallower water depths, and (3) OM in the water column, i.e. from sediment traps. Fresh OM throughout the entire water column showed a relatively constant delta C-13(OM) value of approximately -30.5 parts per thousand. Similar values, about -31 parts per thousand, were obtained for well-varved sediments in both the long and short, sediment-water interface cores. In contrast, delta C-13(OM) values from non-varved sediments in all cores were significantly less negative (-29 parts per thousand). The delta C-13(OM) values in the sediment-water interface cores from different water depths differ for sediments of the same age, if oxygen conditions at the time of deposition were different at these sites, as suggested by the state of varve preservation. Sediments deposited from AD 1924 to 1980 at 62m water depth are varved and exhibit delta C-13(OM) values around -31 parts per thousand, whereas sediments of the same age in the core from 35m water depth are not varved and show less negative delta C-13(OM) values of about -29 parts per thousand. The relation between varve occurrence and delta C-13(OM) values suggests that delta C-13(OM) is associated with oxygen conditions because varve preservation depends on hypolimnetic anoxia. A mechanism that likely influences delta C-13(OM) is selective degradation of OM under oxic conditions, such that organic components with more negative delta C-13(OM) are preferably decomposed, leading to less negative delta C-13(OM) values in the remaining, undegraded OM pool. Greater decomposition of OM in non-varved sediments is supported by lower TOC concentrations in these deposits (similar to 5%) compared to well-varved sediments (similar to 15%). Even in lakes that display small variations in productivity and terrestrial OM input through time, large spatial and temporal differences in hypolimnetic oxygen concentrations may be an important factor controlling sediment delta C-13(OM).
Based on Proxy records from western Black Sea cores, we provide a comprehensive Study of climate change during the last glacial maximum and late-glacial period in the Black Sea region. For the first time we present a record of relative changes in precipitation for NW Anatolia based on variations in the terrigenous supply expressed as detrital carbonate concentration. The good correspondence between reconstructed rainfall intensity in NW Anatolia and past western Mediterranean sea Surface temperatures (SSTs) implies that during the glacial period the precipitation variability was controlled, like today, by Mediterranean cyclonic disturbances. Periods of reduced precipitation correlate well with low SSTs in the Mediterranean related to Heinrich events H1 and H2. Stable oxygen isotopes and lithological and mineralogical data point to a significant modification in the dominant freshwater/sediment source concomitant to the meltwater inflow after 16.4 cal ka BP. This change implies intensification of the northern sediment source and, with other records from the Mediterranean region, consistently suggests a reorganization of the atmospheric circulation pattern affecting the hydrology of the European continent. The early deglacial northward retreat of both atmospheric and oceanic polar fronts was responsible for the warming in the Mediterranean region, leading simultaneously to more humid conditions in central and northern Europe.
The Tibetan Plateau (TP), including its surrounding mountain ranges, represents the largest store of ice outside the polar regions. It hosts numerous lakes as well as the head waters of major Asian rivers, on which billions of people depend, and it is particularly sensitive to climate change. The moisture transport to the TP is controlled by the Indian and Pacific monsoon and the Westerlies. Understanding the evolution of the interaction of these circulation systems requires studies on climate archives in different spatial and temporal contexts. The objective of this study is to learn more about the interannual variability of precipitation patterns across the TP and how different hydrologic systems react to different climatic factors.
Aragonite shells of the aquatic gastropod Radix, which is widely distributed in the region, may represent suitable archives for inferring hydrologic and climatic signals in particularly high resolution. Therefore, sclerochronological studies of delta O-18 and delta C-13 ratios in Radix shells from seven lakes were conducted, each representing a different hydrologic and climatic setting, on a transect from the Pamirs across the TP.
The shell patterns exhibit an increasing influence of precipitation and a decreasing influence of evaporation on the isotope compositions from west to east. delta O-18 values of shells from lakes on the eastern and central TP (Donggi Cona, Yamdrok Yumco, Tarab Co) mirror monsoon signals, indicated by more negative values and higher variabilities compared to the more western lakes (Karakul, Bangong/Nyak, Manasarovar). In Yadang Co, located on the central southern TP, the monsoon rains did not reach the lake in the sampling year, although it is located in a region which is usually affected by monsoon circulation. The delta O-18 values are used to differentiate the annual hydrological cycle into ice cover period, melt water period, precipitation period and evaporation period. delta C-13 compositions in the shells particularly depend on specific habitats, which vary in biological productivity and in carbon sources. delta O-18 and delta C-13 patterns show a positive covariance in shells originating from large closed basins. The results show that Radix shells mirror general climatic differences between the seven lake regions. These differences reflect both regional and local climate signals in sub-seasonal resolution, without noticeable dependence on the particular lake system.
Recognition of an inferred Miocene marine incursion affecting areas from Colombia through Peru and Bolivia and into Argentina is essential to delineate the South American Seaway. In Bolivia, corresponding strata of inferred marine origin have been assigned to the late Miocene Yecua Formation. We carried out high-resolution delta C-13 and delta O-18 isotopic studies on 135 in situ carbonates from 3 outcrops, combined with detailed sedimentologic, paleontologic, and ichnologic analysis. Four less negative delta C-13 excursion levels were recorded that coincide well with beds containing marine body (barnacle) and trace (Ophiomorpha) fossils. These strata are interbedded with red-green beds containing mudcracks, plant roots, gypsum, and trace fossils of the continental Scoyenia ichnofacies. Our data are significant in that they show for the first time four possible short-lived marine incursions in the Bolivian central Andes during the late Miocene. The result is constrained by a new U-Pb date of 7.17 +/- 0.34 Ma at the top of Yecua strata.
The hypersaline crater lake and its catchment on seabird island Isabel (Pacific, off Mexico) was studied to explore the influence of strong seasonal variations in rainfall/evaporation and guano contribution on its limnology. The hypersaline lake water (HSW, 78 %) is up to 2.2-times enriched in inert ions relative to mean seawater. Rainfall during summer dilutes the HSW to form a less saline rainwater body (RWB) above a chemolimnion between 2 and 4 m water depth. The RWB is inhabited first by diatoms and ostracods followed later on by cyanobacteria and ciliates. Evaporation of > 1.5 m depth of lake water over the dry season increases the salinity of the RWB until the water column becomes isohaline at HSW concentrations in the late dry season. Differences in the stable isotope composition of water and primary producers in RWB and HSW reflect this development. Introduction of seabird guano and the decrease of salinity fuel a high primary production in the RWB with higher delta(CDIC)-C-13 and delta(13)Corg of particulate organic matter than in the HSW. The high N supply leads to high delta N-15 NH4 values (+ 39 % in the HSW) as the consequence of ammonia volatilization that is strongest during guano maturation and with evaporative salinity increase from the HSW. Precipitation of carbonate (calcite and aragonite) from the RWB and the HSW is hindered by the high concentration of guano-derived P. This inhibition may be overcome with evaporative supersaturation during particularly dry conditions. Carbonate may also precipitate during particularly wet conditions from the dilute RWB, where the P-concentration is reduced during an active phytoplankton production that raises the pH. Differences in the stable isotope signatures of carbon and oxygen in HSW and RWB (+ 5 % delta(CDIC)-C-13 and -3 % d18OH2O) suggest the processes of carbonate precipitation can be distinguished based on the isotope signature of the carbonates deposited. Changes in the lake system are indicated when lower temperatures and higher rainfall in the 2006 wet season introduced more and less mature guano to the lake. The lower pH was accompanied by lower ammonia volatilization and carbonate precipitation as indicated by an increased concentration of NH4, Ca, Sr and DIC, while delta H-2, delta(NNH4)-N-15, and salinity were lower. According to our results, the observed sediment laminations should reflect the introduction of catchment material (including guano) with runoff, the RWB plankton production, and the carbonate precipitation in relation to its origin and seasonality.
Fixed nitrogen ( N) is a limiting nutrient for algae in the low- latitude ocean, and its oceanic inventory may have been higher during ice ages, thus helping to lower atmospheric CO2 during those intervals. In organic matter within planktonic foraminifera shells in Caribbean Sea sediments, we found that the N-15/N-14 ratio from the last ice age is higher than that from the current interglacial, indicating a higher nitrate N-15/N-14 ratio in the Caribbean thermocline. This change and other species- specific differences are best explained by less N fixation in the Atlantic during the last ice age. The fixation decrease was most likely a response to a known ice age reduction in ocean N loss, and it would have worked to balance the ocean N budget and to curb ice age- interglacial change in the N inventory.
In piston cores from the open subarctic Pacific and the Okhotsk Sea, diatom-bound delta N-15 (delta N-15(db)), biogenic opal, calcium carbonate, and barium were measured from coretop to the previous glacial maximum (MIS 6). Glacial intervals are generally characterized by high delta N-15(db) (similar to 8 parts per thousand) and low productivity, whereas interglacial intervals have a lower delta N-15(db) (5.7-6.3 parts per thousand) and indicate high biogenic productivity. These data extend the regional swath of evidence for nearly complete surface nutrient utilization during glacial maxima, consistent with stronger upper water column stratification throughout the subarctic region during colder intervals. An early deglacial decline in delta N-15(db) of 2 parts per thousand at similar to 17.5 ka, previously observed in the Bering Sea, is found here in the open subarctic Pacific record and arguably also in the Okhotsk, and a case can be made that a similar decrease in delta N-15(db) occurred in both regions at the previous deglaciation as well. The early deglacial delta N-15(db) decrease, best explained by a decrease in surface nutrient utilization, appears synchronous with southern hemisphere-associated deglacial changes and with the Heinrich 1 event in the North Atlantic. This delta N-15(db) decrease may signal the initial deglacial weakening in subarctic North Pacific stratification and/or a deglacial increase in shallow subsurface nitrate concentration. If the former, it would be the North Pacific analogue to the increase in vertical exchange inferred for the Southern Ocean at the time of Heinrich Event 1. In either case, the lack of any clear change in paleoproductivity proxies during this interval would seem to require an early deglacial decrease in the iron-to-nitrate ratio of subsurface nutrient supply or the predominance of light limitation of phytoplankton growth during the deglaciation prior to Bolling-Allerod warming.
This paper aims to highlight the potential of using elemental and stable isotope analyses of aquatic macrophytes in palaeolimnological studies. Potamogeton pectinatus material was collected from modem plants (n=68) and from late glacial and Holocene-aged sediments from Koucha Lake (northeastern Tibetan Plateau; 34.0 degrees N; 97.2 degrees E; 4540 m a.s.l.). It was analyzed for delta C-13(Potamogeton) (modern: -23 to 0 parts per thousand, fossil: -19 to -4 parts per thousand) and delta N-15(Potamogeton) (modern: -11.0 to +13.8 parts per thousand, fossil: -9.5 to +6.7 parts per thousand) in addition to elemental carbon and nitrogen (modem C/N-Potamogeton: 7 to 29; fossil: 13 to 68) and sulfur (fossil: 188-899 mu mol/g dry weight). Fossil data were interpreted in terms of palaeo-nutrient availability and palaeo-productivity based on the modem relationships between various proxies and certain environmental data. Productivity of Potamogeton pectinatus mats at Koucha Lake as indicated by palaeo-epsilon(Potamogeton-TIC) (i.e. the enrichment of delta C-13(Potamogeton) relative to the delta(CTIC)-C-13) was reduced during periods of high conductivity, especially between 10.3 and 7.4 cal kyr BP. Potamogeton pectinatus material from these periods was also characterized by high S-Potamogeton indicating high sulfide concentrations and anoxic conditions within the sediments. However, C/N- Potamogeton ratios and delta N-15(Potamogeton) from the lower core section were found to have been altered by decompositional processes. A pronounced shift in the aquatic productivity of Lake Koucha occurred at similar to 7.4 cal kyr BP when the hydrological conditions shifted towards an open lake system and water depth increased. At this time a strong increase in productivity led to a strong decrease in the water HCO3- concentration as inferred from the application of a epsilon-(Potamogeton-TIC)-InHCO3- transfer function. A comparison of reconstructed productivity changes from Koucha Lake with further environmental proxies suggests that primary productivity changes are probably a function of internal lake dynamics and were only indirectly triggered by climate change.