Journal of Paleolimnology最新文献

We investigated diatom-valve fragmentation and frustule separation in surface sediments in Lake Kitaura, a shallow lake in Japan, to clarify how sedimentation processes affect diatom assemblages. Twenty-two surface-sediment samples were collected from Lake Kitaura in 2014. Aulacoseira spp. were characteristic planktonic taxa, and Cocconeis placentula and Planothidium lanceolatum were characteristic benthic taxa, both with monoraphid valves. The fragmentation ratios of Aulacoseira spp., the most abundant taxon, were higher nearshore than in the central part of the lake. Cocconeis placentula, an epiphytic species, was widely distributed in Lake Kitaura, and its fragmentation ratios increased with distance from the lakeshore. The coexistence ratios of the two valve types of this species decreased with distance from the lakeshore and from macrophytic vegetation. Planothidium lanceolatum, a fluvial benthic species, was abundant at the three northernmost sites near a river mouth, and the frustules of this species were seldom separated. These results suggest that long-distance transport promotes fragmentation and separation of benthic diatom valves in shallow lakes.

Lakes act as natural reservoirs for storing organic material, and comprehending how organic carbon (OC) and black carbon (BC) are deposited in lake sediments is crucial for understanding the global carbon cycle and its impact on climate and ecosystems. In this study, we examined changes in the deposition patterns of OC and BC in Yinjia Lake (YJL) over the past 110 years, using a 60 cm sediment core dated with ²¹⁰Pb. Our aim was to discern how these changes relate to human activities and other influencing factors in the region of southeast Hubei, central China. Our findings revealed a consistent rise in total organic carbon (TOC) and total nitrogen (TN) concentrations, indicating a gradual increase from the bottom upwards. Analysis of C/N ratios and δ¹³C values showed that the OC in the sediment mainly originated from phytoplankton and terrestrial C3 plants. Over the past century, OC burial rates (OCBR) in the YJL core increased from 61.35 to 86.69 g m⁻² yr⁻¹, primarily due to increased primary production resulting from intensified local agriculture and urban growth. Temperature was found to influence OCBR, while precipitation had little impact on OCBR dynamics in sedimentary environments. BC burial rates in YJL ranged from 3.67 to 11.51 g m^–2 yr^–1, significantly exceeding those observed in other lakes worldwide. The fluctuations in BC burial rates correlated with shifts in local industrial practices and energy consumption. In recent years (post-2005), BC burial rates have declined, likely due to reduced pollutant emissions resulting from the implementation of emission-control policies in China. These results provide valuable insights into the interactions between human activities, climate variability, and carbon cycling in lacustrine environments, with implications for regional carbon budgets, ecosystem management, and climate-change mitigation strategies.

This paper is the first one to investigate the stable oxygen isotope values of modern ostracods sampled across the whole area of the Caspian Sea. Six most common taxa (shallow-water Euxinocythere virgata and Tyrrhenocythere amnicola donetziensis; deeper-water Candona schweyeri, Paracyprideis sp., Bakunella dorsoarcuata and Camptocypria sp.) were analyzed for δ¹⁸O_ost. We present a specific isotopic offset for each studied taxa relative to the expected equilibrium value of inorganic calcite. An average vital offset of + 1‰ has been determined. The correlation between δ¹⁸O_ost and some parameters of the aquatic environment was examined. The regression analysis showed correlations to temperature, salinity and δ¹⁸O_water, facilitating the prediction of water mass characteristics when being applied to core sediment records and providing the basis for reliable paleoenvironmental reconstructions in the future using oxygen isotope data. The overall results of 105 δ¹⁸O_ost measurements taken from 76 sampling sites are consistent with the temporal and spatial indicators of the bottom water masses, as impacted by regional hydrography and physical processes. The corresponding inference allows to successfully identify average properties of ambient water mass, where a set of carapaces has been formed, and also allows to recognize the climatical signal in the δ¹⁸O_ost records. The predicted temperatures calculated from the measured δ¹⁸O_ost values correspond to actual measured temperatures at the site of collection, and the oxygen isotope composition of the studied common species can be a reliable indicator of water temperatures for the lake-type steady-state of the Caspian Sea corresponding to the period of shell calcification. However, in the context of the Caspian Sea environmental changes during the Quaternary period, such attempts of calculating paleotemperatures may lead to false results, since the paleotemperature equation and its variations are only applicable to a water body in its steady state. Notwithstanding, the potential of the studied ostracod δ¹⁸O_ost records to be used as a paleoenvironmental proxy in future downcore reconstructions in the Caspian Sea has been shown.

A recent bathymetric survey of Lake Constance revealed ~ 170 mounds composed of loosely deposited rocks aligned in a ~ 10-km long chain along the southern Swiss shoreline in a water depth of 3–5 m. The mounds are 10–30 m in diameter and up to 1.5 m high. Over their entire length of occurrence, the mounds are estimated to be composed of ~ 60 million individual boulders, with a total weight of ~ 78,000 t. A ground penetrating radar (GPR) survey showed that the mounds are not linked to the glacial substrate but were rather deposited artificially on the edge of a prograding shelf composed of Holocene lake sediments. Here, we present the results of a coring campaign with four piston cores along a GPR transect across one of the mounds. The cores recovered the full Holocene sedimentary succession all the way into the basal till that is overlain by lacustrine sediments dating back to ~ 14,400 cal. yrs BP. The four cores are merged into a ~ 12.4-m long composite section reflecting continuous sedimentation from the siliciclastic-dominated Late Glacial to the carbonate-rich Late Holocene. The stratigraphic horizon representing the mound’s construction was radiocarbon-dated to ~ 5600–5300 cal. yrs BP, placing them in the Neolithic period. This age was confirmed by radiocarbon dating of wood samples collected during underwater excavation of the mounds. Geochemical analysis of the Holocene sedimentary succession shows generally high carbonate contents (average of 69%). The interval from 5750 to 4950 cal. yrs BP, a part of the mound period, is characterized by a Holocene minimum in carbonate content (average of 57%) and by larger mean grain sizes. Comparing these values to those from a recent surface-sediment depth transect indicates that this was a period of rather low lake levels, which might have favoured mound construction. Correlations to nearby archaeological sites and to the general West-Central European lake-level record indicates that the mounds likely were built during a short phase of low lake levels during a general trend of climatic cooling followed by a lake-level transgression.

Mining in northern Canada has been known to cause major environmental problems; however, historical monitoring data are scarce or non-existent. Here, we use a multi-proxy (metals, bioindicators, pigments) paleolimnological approach to track the impacts of mining activity near Keno City, on the traditional land of the First Nation of Na-Cho Nyäk Dun, in central Yukon (Canada). Silver was discovered in the early 1900s, primarily on or between two hills (Galena Hill and Keno Hill). Intensive mining has taken place ever since, with brief hiatuses dependent on ore prices and ownership of the claims. Christal Lake, a shallow site located in the valley between both hills, lies near many historical and current mines, and was once the site of a processing mill. Geochemical data show elevated background concentrations of many metals and faithfully track known mining activity. Interestingly, background (pre-mining) sediment concentrations of arsenic, cadmium, and zinc were all elevated above the Canadian Sediment Quality Guidelines for the Protection of Aquatic Life, reflecting the natural weathering of elements in high concentrations from the local catchment. These, and other metal(loid)s, increased and peaked in sedimentary concentration after ca. 1920s, when intensive mining began. Sedimentary chlorophyll-a concentrations declined with the rise of metal concentrations, although values increased again slightly in more recent sediments, perhaps reflecting the decline in recent metal inputs and reclamation of historic mine sites. Meanwhile, subfossil diatom assemblages were dominated by small benthic Fragilaria sensu lato taxa, whose assemblage composition only changed subtly with mining (similar to other shallow, non-acidified sites in the highly metal-impacted area of Norilsk, Siberia). There was no biological evidence of acidification, likely due to the neutralizing effect of the carbonate-rich catchment. Cladoceran subfossils were only present in very low numbers throughout the core, reflecting both the shallow nature and high background metal concentrations in the lake. Collectively, these data show the long-term impacts of silver mining in this subarctic environment.

The Rideau Canal (Ontario, Canada) was constructed in the early 1830s, primarily as a means to transport military personnel, but now is primarily recreational. The construction of the canal and associated flooding, as well as other land-use changes, likely impacted lakes within the system, however, long-term monitoring data are not available. Furthermore, recent environmental changes, including accelerated climate warming, are affecting lake ecosystems. Shallow, macrophyte-dominated Lake Opinicon, which is part of the canal system, has been impacted by other various catchment disturbances over the past ~ 200 years. A previous diatom-based paleolimnological study conducted on the lake, examining a core collected in 1995, found that the diatom responses to a host of large-scale catchment disturbances were moderate compared to nearby deeper lakes. A more recent diatom-based study conducted on a 2019 core (the same core used in the present study) found similar results; however, over the most recent ~ 25–30 years a striking shift in diatom assemblage composition was documented, coinciding with increased regional climate warming. Nothing is known concerning long-term changes within the primary consumers linked to the array of disturbances. Here, we examined changes in cladoceran assemblages over the past ~200 years, using the 2019 sediment core, to track their response to various environmental stressors including climate warming. We found that pelagic Bosmina and Daphnia species began to increase in the early nineteenth century, consistent with the flooding of the lake during canal construction. The most ecologically notable changes in the cladoceran record, however, occurred in the most recent sediments. These were characterized by marked declines (often to trace abundances) in several littoral taxa, concurrent with a further increase in the relative abundance of small, pelagic Bosmina spp. This most recent compositional shift was consistent with accelerated regional climate warming and associated limnological changes, decreased total phosphorus (TP) concentrations, and changing food sources. These changes in primary consumers will likely cascade throughout the food web.

We compare hydrogen isotopic measurements of long-chain leaf-wax n-alkanes (δ²H_w; C₂₇, C₂₉, and C₃₁) from Martin Lake, Indiana, USA, with a calcite-based reconstruction of the oxygen isotopic composition of precipitation (δ¹⁸O_p) from the same lake. We observe stable and high δ²H_w during the Common Era (last 2000 years), which we interpret as growing-season precipitation originating mainly from the Gulf of Mexico and Atlantic. During the Little Ice Age (LIA; 1200–1850 CE), δ²H_w values increased by 3–8 ‰, concomitant with a significant decrease in δ¹⁸O_p values by up to 12.5 ‰. Multiple proxy records for this time indicate persistent growing-season drought. We interpret these relatively high δ²H_w values, as compared to the δ¹⁸O_p values, as a signal of low relative humidity that resulted in an ²H enrichment in plant source water resulting in high δ²H values through enhanced plant water and/or soil evaporation. These results support the occurrence of low humidity conditions during the LIA in the midcontinental USA that also contributed to the marked decline of regional pre-Columbian Mississippian populations.

For long time in the history of Earth, ferruginous conditions governed the oceans. With the rise of oxygen during the Proterozoic era and the subsequent evolution of living organisms, worldwide deposition of iron formations occurred. These sedimentary units reveal the transition into oxic oceans, passing by local and transitory euxinic conditions, especially in coastal shelves. Constraining the iron cycle and the biogeochemical processes occurring in present and past ferruginous basins helps answering some of the question regarding global oxygenation, the evolution of life and past climate changes. Therefore, Fe speciation and Fe isotopes in both Proterozoic and recent sedimentary records have been widely used to reconstruct past basin dynamics and redox conditions in the sediment–water interface. However, sedimentation and early diagenesis can alter paleoredox proxies and their primary climate signals. In this work, we disentangled alteration processes occurring at the redox front below the sediment–water interface of a ventilated deep-water lake (Lago Fagnano, Argentina/Chile). A sequential extraction protocol was applied to characterize two reactive Fe pools: Fe oxyhydroxides and reduced iron. Subsequently, Fe isotopes were constrained to determine the main processes mobilizing Fe. At the redox front, ferric minerals reach a δ⁵⁶Fe value of − 1.3‰ resulting from oxidation of dissolved Fe likely following a Rayleigh distillation effect. Dissolved Fe is produced right below via Fe reduction, as shown by the low ferric Fe content. Our observations delineate a redox cycle and a redox horizon undergoing constant upward migration, initiated by regular sedimentation. However, during events of increased rapid sedimentation (e.g., seismites) this dynamic cycle is interrupted inducing full or partial preservation of the Fe-rich redox front. In such case, oxidation of dissolved Fe is interrupted and can be recycled in ferrous minerals, such as Fe monosulfides and amorphous phases with δ⁵⁶Fe values down to − 1.7 ‰. These findings have significant implications for the recording of biogeochemical cycles in the geological past, the use of Fe isotopes in freshwater-lake sediments for paleoclimate studies, and the progress of our knowledge regarding the geochemistry of past oceans.