Strontium Isotopic Signatures of the Streams and Lakes of Taylor Valley, Southern Victoria Land, Antarctica: Chemical Weathering in a Polar Climate

We have collected and analyzed a series of water samples from three closed-basin lakes (Lakes Bonney, Fryxell, and Hoare) in Taylor Valley, Antarctica, and the streams that flow into them. In all three lakes, the hypolimnetic waters have different ⁸⁷Sr/⁸⁶Sr ratios than the surface waters, with the deep water of Lakes Fryxell and Hoare being less radiogenic than the surface waters. The opposite occurs in Lake Bonney. The Lake Fryxell isotopic ratios are lower than modern-day ocean water and most of the whole-rock ratios of the surrounding geologic materials. A conceivable source of Sr to the system could be either the Cenozoic volcanic rocks that make up a small portion of the till deposited in the valley during the Last Glacial Maximum or from marble derived from the local basement rocks. The more radiogenic ratios from Lake Bonney originate from ancient salt deposits that flow into the lake from Taylor Glacier and the weathering of minerals with more radiogenic Sr isotopic ratios within the tills. The Sr isotopic data from the streams and lakes of Taylor Valley strongly support the notion documented by previous investigators that chemical weathering has been, and is currently, a major process in determining the overall aquatic chemistry of these lakes in this polar desert environment.     

Sedimentological proof and dating of the Early Holocene volcanic eruption of Ulmener Maar (Vulkaneifel,Germany)

A minerogeniclayer occurs in early postglacial organic sediments from five maar lakes (West Eifel Volcanic Field, Germany). The mineralogy and stratigraphic position of this tephra suggests that it is related to the youngest German volcano, Ulmener Maar, nearby. Radiocarbon dating of wood from the base of the Ulmener Maar Tephra at two locations provide ages in agreement with an accelerator mass spectrometer ¹⁴C date for the minerogenic layer from sediments of Lake Holzmaar situated 13 km south-west of Ulmener Maar. The mean radiocarbon age is 9 560 years BP. Dating by varve chronology provides an age of 10017 years VT (varve time in years before 1950) or 10 895 years corrected VT. Based on palynology the Ulmener Maar Tephra was deposited at the end of the Preboreal. High values of natural remnant magnetization intensity, typical of pyroclastic material, confirm that this minerogenic layer differs in composition from other clastic deposits of the sedimentary record. Geochemical analyses reveal increased values of total trace elements for the Laacher See Tephra and Ulmener Maar Tephra. An isopach map based on thickness variations of the Ulmener Maar Tephra at five investigated maar lakes indicates that the tephra was mainly transported to the south west.     

Constraints from strontium and neodymium isotopic ratios and trace elements on the sources of the sediments in Lake Huguang Maar

The sediments in Lake Huguang Maar in coastal South China were previously thought to originate mainly from wind-blown dust transported from North China, such that the lake sediments recorded the varying strength of the Asian winter monsoon. An alternative explanation was that the local pyroclastic rocks supplied the lake sediments, but the actual contributions from the different sources remained unclear. Geochemical analyses including ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd and trace elements support the local pyroclastic rock as the dominant source: < 22% of the total Sr in the lake sediments and  17% of the Nd arises from the distant source. Nb/Ta and Zr/Hf for the lake sediments are identical to those for the local rock but differ from the ratios for the wind-blown dust, and chondrite-normalized rare earth element patterns for the lake sediments are similar to those for the local rock and soil, but differ from those for the distant source. The sediments in Lake Huguang Maar are probably input into the lake through runoff and thus controlled by the hydrology of the lake. Wind-blown dust transported by the Asian winter monsoon from arid North China is only a minor contribution to the sediments.     

(Pre-) historic changes in natural and anthropogenic heavy metals deposition inferred from two contrasting Swiss Alpine lakes

Continuous high-resolution sedimentary record of heavy metals (chromium (Cr), copper (Cu), lead (Pb), zinc (Zn), manganese (Mn), and mercury (Hg)), from lakes Lucerne and Meidsee (Switzerland), provides pollutant deposition history from two contrasting Alpine environments over the last millennia. The distribution of conservative elements (thorium (Th), scandium (Sc) and titanium (Ti)) shows that in absence of human disturbances, the trace element input is primarily controlled by weathering processes (i.e., runoff and erosion). Nonetheless, the enrichment factor (EF) of Pb and Hg (that are measured by independent methods), and the Pb isotopic composition of sediments from the remote lake Meidsee (which are proportionally more enriched in anthropogenic heavy metals), likely detect early mining activities during the Bronze Age. Meanwhile, the deposition of trace elements remains close to the range of natural variations until the strong impact of Roman activities on atmospheric metal emissions. Both sites display simultaneous increases in anthropogenic trace metal deposition during the Greek and Roman Empires (ca 300 BC to AD 400), the Late Middle Ages (ca AD 1400), and the Early Modern Europe (after ca AD 1600). However, the greatest increases in anthropogenic metal pollution are evidenced after the industrial revolution of ca AD 1850, at low and high altitudes. During the twentieth century, industrial releases multiplied by ca 10 times heavy metal fluxes to hydrological systems located on both sides of the Alps. During the last decades, the recent growing contribution of low radiogenic Pb further highlights the contribution of industrial sources with respect to wood and coal burning emissions.     

Atmospheric Pb-pollution by pre-medieval mining detected in the sediments of the brackish karst lake An Loch Mór,western Ireland

This paper presents results of geochemical investigations of lake sediments from the karst lake An Loch Mór, Aran Islands, including the first highly resolved record of atmospheric Roman Pb pollution for Ireland. The natural Pb influx into the lake is largely contributed by 3 Pb components, which differ in their isotopic composition: detrital influx of Pb from the siliciclastic input, dissolved influx of Pb released by weathering of the local limestone, and dissolved influx of seawater Pb. The balance between the 3 Pb components varies in concert with the hydrological evolution of the lake. The influx of Pb in dissolved form is estimated by geochemical mass balance assuming that the siliciclastic influx is characterised by the Pb/Al-ratio of the Late Glacial clastic sediments. It typically accounts for 50–80% of total Pb input in the Holocene sediments of An Loch Mór. The natural dissolved influxes of Pb, Sc, and Y reach a similar order of magnitude. Normalisation with Sc and Y is applied to quantify contributions from anthropogenic Pb. Based on continuous sampling of 1 cm sample slices, variations in the influx of Roman Pb could be reconstructed at a time resolution of c. 5 a. Combined geochemical and Pb isotope mass balance is used to characterise the isotopic composition of anthropogenic Pb. Distinctly enhanced influx of anthropogenic Pb occurs in the 1st and 2nd century AD and shows high variability on decadal scale. This is in contrast to central European Pb records, which document a gradual increase and decrease in ancient atmospheric pollution by Roman Pb. The reconstructed high variability in the influx of Roman Pb in An Loch Mór documents variations in the wind regime of western Europe, temporarily favouring the transport of atmospheric Pb to western Ireland.     

A 2400-year record of trace metal loading in lake sediments of Lagoa Vermelha, southeastern Brazil

Sediments of the Lagoa Vermelha (Red Lake), situated in the Ribeira Valley, southeastern Brazil, are made of a homogeneous, organic-rich, black clay with no visible sedimentary structures. The inorganic geochemical record (Al, As, Ba, Br, Co,Cs, Cr, Fe, Mn, Ni, Rb, Sc, Sb, V, Zn, Hg and Pb) of the lake sediments was analyzed in a core spanning 2430 years. The largest temporal changes in trace metal contents occurred approximately within the last 180 years. Recent sediments were found to be enriched in Pb, Zn, Hg, Ni, Mn, Br and Sb (more than 2-fold increase with respect to the “natural background level”). The enhanced accumulation of Br, Sb, and Mn was attributed to biogeochemical processes and diagenesis. On the other hand, the anomalous concentrations of Pb, Zn, Hg and Ni were attributed to pollution. As Lagoa Vermelha is located in a relatively pristine area, far removed from direct contamination sources, the increased metal contents of surface sediments most likely resulted from atmospheric fallout. Stable Pb isotopes provided additional evidence for anthropogenic contamination. The shift of ²⁰⁶Pb/²⁰⁷Pb ratios toward decreasing values in the increasingly younger sediments is consistent with an increasing contribution of airborne anthropogenic lead. In the uppermost sediments (0-10 cm), the lowest values of the ²⁰⁶Pb/²⁰⁷Pb ratios may reflect the influence of the less radiogenic Pb from the Ribeira Valley District ores (²⁰⁶Pb/²⁰⁷Pb between 1.04 and 1.10), emitted during the last 50 years.     

Identification of contamination in a lake sediment core using Hg and Pb isotopic compositions,Lake Ballinger,Washington, USA

Concentrations and isotopic compositions of Hg and Pb were measured in a sediment core collected from Lake Ballinger, near Seattle, Washington, USA. Lake Ballinger has been affected by input of metal contaminants emitted from the Tacoma smelter, which operated from 1887 to 1986 and was located about 53 km south of the lake. Concentrations and loadings of Hg and Pb in Lake Ballinger increased by as much as three orders of magnitude during the period of smelting as compared to the pre-smelting period. Concentrations and loadings of Hg and Pb then decreased by about 55% and 75%, respectively, after smelting ended. Isotopic compositions of Hg changed considerably during the period of smelting (δ²⁰²Hg = −2.29‰ to −0.38‰, mean −1.23‰, n = 9) compared to the pre-smelting period (δ²⁰²Hg = −2.91‰ to −2.50‰, mean −2.75‰, n = 4). Variations were also observed in ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb isotopic compositions during these periods. Data for Δ¹⁹⁹Hg and Δ²⁰¹Hg indicate mass independent fractionation (MIF) of Hg isotopes in Lake Ballinger sediment during the smelting and post-smelting period and suggest MIF in the ore smelted, during the smelting process, or chemical modification at some point in the past. Negative values for Δ¹⁹⁹Hg and Δ²⁰¹Hg for the pre-smelting period are similar to those previously reported for soil, peat, and lichen, likely suggesting some component of atmospheric Hg. Variations in the concentrations and isotopic compositions of Hg and Pb were useful in tracing contaminant sources and the understanding of the depositional history of sedimentation in Lake Ballinger.     

The late Quaternary limnological history of Lake Kinneret (Sea of Galilee), Israel

The freshwater Lake Kinneret (Sea of Galilee) and the hypersaline Dead Sea are remnant lakes, evolved from ancient water bodies that filled the tectonic depressions along the Dead Sea Transform (DST) during the Neogene-Quartenary periods. We reconstructed the limnological history (level and composition) of Lake Kinneret during the past ∼40,000 years and compared it with the history of the contemporaneous Lake Lisan from the aspect of the regional and global climate history. The lake level reconstruction was achieved through a chronological and sedimentological investigation of exposed sedimentary sections in the Kinnarot basin trenches and cores drilled at the Ohalo II archeological site. Shoreline chronology was established by radiocarbon dating of organic remains and of Melanopsis shells.The major changes in Lake Kinneret level were synchronous with those of the southern Lake Lisan. Both lakes dropped significantly ∼42,000, ∼30,000, 23,800, and 13,000 yr ago and rose ∼39,000, 26,000, 5000, and 1600 yr ago. Between 26,000 and 24,000 yr ago, the lakes merged into a unified water body and lake level achieved its maximum stand of ∼170 m below mean sea level (m bsl). Nevertheless, the fresh and saline water properties of Lake Kinneret and Lake Lisan, respectively, have been preserved throughout the 40,000 years studied. Calcium carbonate was always deposited as calcite in Lake Kinneret and as aragonite in Lake Lisan-Dead Sea, indicating that the Dead Sea brine (which supports aragonite production) never reached or affected Lake Kinneret, even during the period of lake high stand and convergence. The synchronous level fluctuation of lakes Kinneret, Lisan, and the Holocene Dead Sea is consistent with the dominance of the Atlantic-Mediterranean rain system on the catchment of the basin and the regional hydrology. The major drops in Lake Kinneret-Lisan levels coincide with the timing of cold spells in the North Atlantic that caused a shut down of rains in the East Mediterranean and the lakes drainage area.     

Lead in prehistoric,historic and contemporary Japanese: stable isotopic study by ICP mass spectrometry

Lead concentration and isotopic composition of prehistoric (middle and latest Jomon era, 2000–4500 BP, n=6), historic (Edo era, 130–400 BP, n=10), and contemporary (died in 1987–88, n=15) Japanese bones, and deciduous teeth from contemporary Japanese children born during 1985–88 (n=17) were analyzed by inductively coupled plasma mass spectrometry. Lead concentration was lowest in Jomon bones and was higher in rural Edo, contemporary, and urban Edo, in that order. Elevated Pb concentration in historic Edo people, as reported previously, was reconfirmed. The average isotopic ratios (²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb) in excavated (prehistoric and historic) bones, contemporary bones, and deciduous teeth were different from each other. The contemporary bones had the least radiogenic composition (mean ²⁰⁷Pb/²⁰⁶Pb: 0.879; mean ²⁰⁸Pb/²⁰⁶Pb: 2.126) while the excavated bones the most (0.848; 2.098), and teeth intermediate (0.866; 2.111). The comparison with the literature data of isotopic compositions of environmental samples showed that the isotopic composition of the excavated bones was within the range of Japanese ores, rocks and soils, indicating the absence of foreign Pb sources in preindustrialized Japan. That of the contemporary bones was closer to the average gasoline Pb, the use of which had been banned in the late 1970s, than to the Pb in airborne particulate matter or refuse incineration ash of 1980s. The average Pb isotopic ratios in the deciduous teeth was close to the isotopic ratios of Pb in airborne particulate matter and refuse incineration ash. These data indicated that the contemporary Japanese population was exposed to foreign Pb which had different isotopic composition from domestic Pb. Exposure to Pb of foreign origin was particularly evidently recorded in people born before the leaded gasoline ban. The history of human Pb contamination in Japan is discussed based on the present results and other previously published data.     

Spatial distribution and flux of terrigenic He dissolved in the sediment pore water of Lake Van (Turkey)

In this study, the largest ever carried out to measure noble gases in the pore water of unconsolidated sediments in lakes, the emission of terrigenic He through the sediment column of Lake Van was successfully mapped on the local scale. The main input of He to the water body occurs at the borders of a deep basin within the lake, which is probably the remains of a collapsed caldera. The ³He/⁴He ratio identifies the He injected into the sedimentary column of Lake Van as a mixture of He released from a mantle source and radiogenic He of crustal origin (³He/⁴He∼2.6-4.1×10^-6). During passage through the pore space, terrigenic He seems to be further enriched in radiogenic He that is most likely produced in the sediment column. In fact, two distinct trends in isotopic composition can be distinguished in the He injected from the lake basement into the sediments. One of these characterizes samples from the shallow water, the other characterizes samples from the deep basin. However, both of these trends are related to the same source of terrigenic He. The He fluxes determined seem to be characteristic of each sampling location and might be considered as a proxy for the fluid permeability of the deep sediment column. These new findings provide insight into the process of fluid transport within the sediments and into the process of formation of the lake basin. Moreover, the isotopic signature of the He that emanates into the water column of Lake Van is strongly affected by the mixing conditions prevailing in the overlying water body. This fact misled previous studies to interpret the terrigenic He in Lake Van as being solely of mantle origin (³He/⁴He∼10^-5).     

Determination of the oxidation state of radiogenic Pb in natural zircon using X-ray absorption near-edge structure

We examined the L_III-edge Pb X-ray absorption near-edge structure (XANES) of three natural zircon samples with different amounts of radiation doses (1.9 × 10¹⁵ to 6.8 × 10¹⁵ α-decay events/mg). The results suggest that the oxidation state of radiogenic Pb in the zircon sample with the highest radiation dose is divalent. The XANES spectra of the two other samples with lower radiation doses suggest that radiogenic Pb(II) is present, and further that some Pb may be tetravalent. This is the first work on the determination of the oxidation state of radiogenic Pb in natural zircon using XANES.     

Seasonal variations of 210Pb and 210Po concentrations in an oligotrophic lake

The vertical distribution of ²¹⁰Pb and ²¹⁰Po in the dissolved (<0.4 μm) and the paniculate (>0.4 μm) phases was measured in Crystal Lake, Wisconsin, to examine the spatial and temporal variability during the seasonal cycle of this oligotrophic lake. The concentration of unsupported ²¹⁰Pb in the water column is maintained principally by atmospheric input. However, most of the ²¹⁰Po in Crystal Lake is produced in situ from radioactive decay of ²¹⁰Pb.Mass balance considerations indicated that the removal rates of ²¹⁰Pb and ²¹⁰Po from the water column to the sediment varied temporally by nearly an order of magnitude. During transient periods of high biological productivity, a large net flux of these nuclides into the sediment occurred. In addition, ²¹⁰Pb was rapidly stripped from the water column during fall turnover. It was during these short-lived events that most of the annual net removal of ²¹⁰Pb and ²¹⁰Po occurred. The mean removal residence time was estimated to be 0.095 yr for ²¹⁰Pb and 0.26 yr for ²¹⁰Po. These residence times suggest that there is a difference between ²¹⁰Pb and ²¹⁰Po in the extent of their recycling in the water column. Calculations indicated that there was a cyclic response of the water column ²¹⁰Po inventory corresponding to successive time periods where there was a net loss or net gain. This cycling is attributed to rapid biological removal and subsequent release from the sediment of freshly deposited ²¹⁰Po. For ²¹⁰Pb, replenishment of the water column appeared to occur mainly from atmospheric input.     

Primary carbonates and Ca-chloride brines as monitors of a paleo-hydrological regime in the Dead Sea basin

Lakes Samra, Lisan and the Dead Sea occupied the Dead Sea basin during the Last Interglacial (∼140–75 ka BP), last glacial (∼70–14 ka BP) and Holocene periods, respectively. The age of Lake Lisan and Samra was determined by U–Th dating of primary aragonites comprising parts of the lacustrine sedimentary sequences. The lakes have periodically deposited sequences of layered calcitic marls (Lake Samra) or laminated primary aragonite (Lake Lisan). The deposition of aragonite as the primary carbonate phase reflects the contribution of the incoming freshwater (loaded with bi-carbonate) and high Mg-, Ca-chloride brine that originated from the subsurface vicinity of the Dead Sea basin. Deposition of calcitic marls suggests a minor effect of the brines. The Ca-chloride subsurface brine has been migrating in and out of the wall rocks of the Dead Sea basin, reflecting the regional hydrological conditions. During most of the last glacial period and during the late Holocene, sufficient precipitation above the Judea Mountains pushed the subsurface Ca-chloride brines into the lakes causing the deposition of aragonite. During the Last Interglacial period the rain that precipitated above the Judea Mountains was insufficient to induce brine flow toward Lake Samra. It appears that sporadic floods provided calcium, bicarbonate and detritus to produce the Samra calcitic marls. Travertines deposited at the Samra–Lisan boundary indicate the early stage in the resumption of groundwater (springs) activity that led to the resurgence of Ca-chloride brine and rise of Lake Lisan. Similar variations in the regional rain precipitation and hydrological activity probably characterized the long-term geochemical evolution of Pleistocene lacustrine water-bodies in the Dead Sea basin, enabling the use of the carbonates as paleo-hydrological monitors.     

Pollution records from sediments of three lakes in New York State

Sediment core segments from Sylvan Lake, Lake Champlain and Lake Canadarago were dated radiometrically with ²¹⁰Pb and ¹³⁷Cs. Their respective sedimentation rates were determined to be 0.11, 0.14 and 0.52 g cm^?2 yr^?1. For the two lakes of lower sedimentation the variations of selected elemental abundances as function of depth were analyzed. Two groupings were found: Al, K, Ti, Rb and Zr were correlated among themselves but reflected different variations in the input of terrigenous erosion material to the lakes. The Cu, Zn and Pb correlated among themselves showed similar depth dependence with increasing concentrations toward the top which can be attributed to cultural pollution. Recent ‘excess’ fluxes to the sediments above the natural contribution by clastic material were derived for the location of the cores, which for Cu, Zn and Pb amounted to 3.8, 24 and 16 μg cm^?2 yr^?1 respectively for Sylvan Lake and 4.9, 20 and 16 μg cm^?2 yr^?1 for Lake Champlain. The corresponding ²¹⁰Pb flux was 3.3 and 2.3 dpm cm^?2 yr^?1, respectively for the two lakes.Approximate residence times in the water column were obtained for trace metals at the Lake Champlain location. Short residence times estimated for Pb (< 0.15 yr) and Cu (< 0.4 yr) indicate fast removal, whereas those for Zn (1.0 ± 0.3 yr) and Cr (2.0 ± 0.5 yr) appeared to be dominated by the water residence time.     

Regional variations in atmospheric deposition and sources of anthropogenic lead in lake sediments across the Canadian Arctic

Stable Pb isotope profiles in dated lake sediment cores were used to gauge the relative amounts and possible sources of anthropogenic Pb deposited from the atmosphere in different regions of the Canadian Arctic. A distinct north-south difference was found. In four High Arctic lakes (i.e., north of 66°N) in this study, recent Pb isotopic shifts or concentration increases attributable to anthropogenic Pb were negligible. The maximum possible contribution from anthropogenic Pb was 0 to 19% of acid-leachable Pb in the 1980s or 1990s. In contrast, two lakes in the Hudson Bay region displayed significantly lower Pb isotope ratios and threefold to fivefold increases of Pb concentrations in modern sediments, corresponding to anthropogenic Pb inputs of at least 72 to 91% of leachable Pb. Eurasian urban and industrial Pb is known to dominate the High Arctic atmosphere. A possible explanation for its negligible influence on northern lake sediments is that atmospheric Pb deposition at northern latitudes is reduced compared with southern regions and is small compared to local geological inputs. ²¹⁰Pb deposition declines with increasing latitude, apparently because of declining precipitation rates; stable Pb deposition may be similarly affected. Meteorological considerations and variations in the post-1900 Pb isotopic trajectories indicated that the predominant anthropogenic Pb source region in NW Hudson Bay was Eurasia, while in SE Hudson Bay, it was Canada and the United States, with a minor Eurasian component.     

Geochemical variations in the Cocos Plate subducting beneath Central America: implications for the composition of arc volcanism and the extent of the Galápagos Hotspot influence on the Cocos oceanic crust

New geochemical data from the Cocos Plate constrain the composition of the input into the Central American subduction zone and demonstrate the extent of influence of the Galápagos Hotspot on the Cocos Plate. Samples include sediments and basalts from Ocean Drilling Program (ODP) Site 1256 outboard of Nicaragua, gabbroic sills from ODP Sites 1039 and 1040, tholeiitic glasses from the Fisher Ridge off northwest Costa Rica, and basalts from the Galápagos Hotspot Track outboard of Central Costa Rica. Site 1256 basalts range from normal to enriched MORB in incompatible elements and have Pb and Nd isotopic compositions within the East Pacific Rise MORB field. The sediments have similar ²⁰⁶Pb/²⁰⁴Pb and only slightly more radiogenic ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb isotope ratios than the basalts. Altered samples from the subducting Galápagos Hotspot Track have similar Nd and Pb isotopic compositions to fresh Galápagos samples but have significantly higher Sr isotopic composition, indicating that the subduction input will have a distinct geochemical signature from Galápagos-type mantle material that may be present in the wedge beneath Costa Rica. Gabbroic sills from Sites 1039 and 1040 in East Pacific Rise (EPR) crust show evidence for influence of the Galápagos Hotspot ∼100 km beyond the morphological hotspot track.     

Sediment storage and severity of contamination in a shallow reservoir affected by historical lead and zinc mining

A combination of sediment-thickness measurement and bottom-sediment coring was used to investigate sediment storage and severity of contamination in Empire Lake (Kansas), a shallow reservoir affected by historical Pb and Zn mining. Cd, Pb, and Zn concentrations in the contaminated bottom sediment typically exceeded baseline concentrations by at least an order of magnitude. Moreover, the concentrations of Cd, Pb, and Zn typically far exceeded probable-effects guidelines, which represent the concentrations above which toxic biological effects usually or frequently occur. Despite a pre-1954 decrease in sediment concentrations likely related to the end of major mining activity upstream by about 1920, concentrations have remained relatively stable and persistently greater than the probable-effects guidelines for at least the last 50 years. Cesium-137 evidence from sediment cores indicated that most of the bottom sediment in the reservoir was deposited prior to 1954. Thus, the ability of the reservoir to store the contaminated sediment has declined over time. Because of the limited storage capacity, Empire Lake likely is a net source of contaminated sediment during high-inflow periods. The contaminated sediment that passes through, or originates from, Empire Lake will be deposited in downstream environments likely as far as Grand Lake O’ the Cherokees (Oklahoma).     

Vegetation and climate changes over the last 30 000 years on the Leizhou Peninsula,southern China,inferred from the pollen record of Huguangyan Maar Lake

A pollen record from Huguangyan Maar Lake documents regional palaeovegetation and palaeoclimate changes in southern China over the last 30 000 years. Huguangyan Maar Lake is located close to the South China Sea (SCS) coastline and is influenced by the East Asian Monsoon (EAM). The pollen assemblages show a succession of vegetation and climate changes. During the Last Glaciation, 30–15.8 cal. ka BP, the Huguangyan area was dominated by subtropical evergreen‐deciduous forest with grassland surrounding the lake, indicating a colder and drier climate than today. During 15.8–11 cal. ka BP, the study area experienced several climatic fluctuations. From 11 to 2 cal. ka BP, the climate shifted to warmer and wetter conditions. After the Holocene Optimum in the early Holocene, the temperature and precipitation decreased. The sediment record of the last 2000 years cannot be used to interpret natural palaeoclimate changes due to the intense anthropogenic influences. Overall, however, the Huguangyan pollen archive highlights the rapid responses of subtropical vegetation to insolation changes in southern China.     

Sr–Nd–Hf–Pb isotope geochemistry of basaltic rocks from the Cretaceous Gyeongsang Basin,South Korea: Implications for basin formation

To better understand the formative mechanism of the Cretaceous Gyeongsang Basin in South Korea, we determined the geochemical compositions of Early Cretaceous syntectonic basaltic rocks intercalated with basin sedimentary assemblages. Two distinct compositional groups appeared: tholeiitic to calc-alkaline basalts from the Yeongyang sub-basin and high-K to shoshonitic basaltic trachyandesites from the Jinju and Uiseong sub-basins. All collected samples exhibit patterns of light rare earth element enrichment and chondrite-normalized (La/Yb)_N ratios ranging from 2.4 to 23.6. In a primitive-mantle-normalized spidergram, the samples show distinctive negative anomalies in Nb, Ta, and Ti and a positive anomaly in Pb. The basalts exhibit no or a weak positive U anomaly in a spidergram, but the basaltic trachyandesites show a negative U anomaly. The basalts have highly radiogenic Sr [(⁸⁷Sr/⁸⁶Sr)_i = 0.70722–0.71145], slightly negative ε_Nd, positive ε_Hf [(ε_Nd)_i = −2.7 to 0.0; (ε_Hf)_i = +2.9 to +6.4], and radiogenic Pb isotopic compositions [(²⁰⁶Pb/²⁰⁴Pb)_i = 18.20–19.19; (²⁰⁷Pb/²⁰⁴Pb)_i = 15.60–15.77; (²⁰⁸Pb/²⁰⁴Pb)_i = 38.38–39.11]. The basaltic trachyandesites are characterized by radiogenic Sr [(⁸⁷Sr/⁸⁶Sr)_i = 0.70576–0.71119] and unradiogenic Nd, Hf, and Pb isotopic compositions [(ε_Nd)_i = −14.0 to −1.4; (ε_Hf)_i = −17.9 to +3.7; (²⁰⁶Pb/²⁰⁴Pb)_i = 17.83–18.25; (²⁰⁷Pb/²⁰⁴Pb)_i = 15.57–15.63; (²⁰⁸Pb/²⁰⁴Pb)_i = 38.20–38.70]. The “crust-like” signatures, such as negative Nb–Ta anomalies, elevated Sr isotopic compositions, and negative ε_Nd(t) and ε_Hf(t) values, of the basaltic trachyandesites resemble the geochemistry of Early Cretaceous mafic volcanic rocks from the southern portion of the eastern North China Craton. Considering the lower-crust-like low U/Pb and high Th/U ratios and the unradiogenic Pb isotopic compositions, the basaltic trachyandesites are considered to be derived from lithospheric mantle modified by interaction with melts that originated from foundered eclogite. Basaltic volcanism in the Yeongyang sub-basin is coeval with the basaltic trachyandesite magmatism, but it exhibits an elevated ⁸⁷Sr/⁸⁶Sr ratio at a given ¹⁴³Nd/¹⁴⁴Nd and highly radiogenic Pb isotopic compositions, which imply an origin from an enriched but heterogeneous lithospheric mantle source. Melts from subducted altered oceanic basalt and pelagic sediments are considered to be the most likely source for the metasomatism. An extensional tectonic regime induced by highly oblique subduction of the Izanagi Plate beneath the eastern Asian margin during the Early Cretaceous might have triggered the opening of the Gyeongsang Basin. Lithospheric thinning and the resultant thermal effect of asthenospheric upwelling could have caused melting of the metasomatized lithospheric mantle, producing the Early Cretaceous basaltic volcanism in the Gyeongsang Basin.