Eutrophication and contamination dynamics of Schweriner See, NE-Germany, during the past 670 years - A multi-proxy approach on lacustrine surface sediments and sediment cores

A multi-proxy study on sedimentary records from Schweriner See (See = lake), NE-Germany, covering the past 670 years (from 1350 CE to today) combined with sediment surface samples to better understand lake internal dynamics enables to reconstruct local and supra-regional eutrophication and contamination trends. Our approach shows that a thorough understanding of depositional processes is crucial for core site selection since at Schweriner See wave- and wind-induced processes in shallow water areas (e.g. reworking) or carbonate precipitation resulting from groundwater inflow may have altered the desired (in this case anthropogenic) signal. In Schweriner See both eutrophication and contamination have been directly influenced by sewage and population dynamics of the city of Schwerin and its surroundings. A higher population density resulted in increased sewage volume, which was discharged directly into Schweriner See since 1893 CE. Maximum eutrophication was reached in the 1970s but a solid improvement in water quality only occurred after the German reunification (1990) as a combined result of a decrease in population density and the connection of all households to a new sewage treatment plant, which stopped the discharge of sewage waters into Schweriner See. These counter measurements were traced within the sediment records. Eutrophication and contamination trends were detected within the lake basin as shown by remarkable similarities in signals between several sediment cores. To get an understanding of regional contamination tendencies east of the former inner German border in the recent past we compared our results with sediment records from the southern Baltic Sea area, which show similar contamination trends.

Organochlorine pesticide contamination in sediments from Richards Bay, South Africa: spatial trends and ecotoxicological risks

The occurrence and distribution of organochlorine pesticides (OCPs) in aquatic systems is a matter of global concern and poses significant toxicological threats to both organisms and human health. Despite the extensive use of OCPs for pest and disease control in southern Africa, relatively few studies have examined the occurrence and toxicological risks of OCP residues in the region. This study investigates the composition, distribution, and potential sources of OCP contamination in sediments from Richards Bay, a rapidly developing industrial port on the northeast coast of South Africa. Surface sediments collected from Richards Bay Harbour and surrounding areas indicate that OCP contamination in the region is widespread. Total concentrations (∑OCP) in surface samples ranged from 135 to 1020 ng g^-1, with hexachlorocyclohexanes (∑HCH; 35-230 ng g^-1) and dichlorodiphenyltrichloroethanes (∑DDT; 12-350 ng g^-1) the dominant contaminant groups detected. Metabolite isomeric compositions indicate that the presence of aldrin and endosulfan likely result from historical agricultural usage, while recent input of γ-HCH, heptachlor, and endrin may be linked to the illegal use of old pesticide stockpiles. Total DDT concentrations were dominated by p,p'-DDT (80 ± 64 ng g^-1), which was attributed to its ongoing use in malaria vector control in the region. A ²¹⁰Pb-dated sediment core revealed that OCP input to the local environment increased dramatically from relatively low concentrations in the mid-1940s (∑OCP, 355 ng g^-1) to peak levels (∑OCP, 781 ng g^-1) in the 1980s/1990s. An overall decrease in ∑OCP concentration from the mid-2000s is likely related to restrictions on use following the Stockholm Convention in 2004. Despite current restrictions on use, OCP concentrations exceeded sediment quality guidelines in the vast majority of cases, raising concerns for protected estuarine and mangrove habitats in the area, as well as for local fishing and farming communities.

Rain Forest Fragmentation and Environmental Dynamics on Nosy Be Island (NW Madagascar) at 1300 cal BP Is Attributable to Intensified Human Impact

Madagascar houses one of the Earth’s biologically richest, but also one of most endangered, terrestrial ecoregions. Although it is obvious that humans substantially altered the natural ecosystems during the past decades, the timing of arrival of early inhabitants on Madagascar as well as their environmental impact is still intensively debated. This research aims to study the beginning of early human impact on Malagasy natural ecosystems, specifically on Nosy Be island (NW Madagascar) by targeting the sedimentary archive of Lake Amparihibe, an ancient volcanic crater. Based on pollen, fungal spore, other non-pollen palynomorph, charcoal particle and diatom analyses combined with high-resolution sediment-physical and (in)organic geochemical data, paleoenvironmental dynamics during the past three millennia were reconstructed. Results indicate a major environmental change at ca. 1300 cal BP characterized by an abrupt development of grass (C4) dominated and fire disturbed landscape showing the alteration of natural rain forest. Further, increased soil erodibility is suggested by distinct increase in sediment accumulation rates, a strong pulse of nutrient input, higher water turbidity and contemporaneous increase in spores of mycorrhizal fungi. These parameters are interpreted to show a strong early anthropogenic transformation of the landscape from rain forest to open grassland. After ca. 1000 cal BP, fires remain frequent and vegetation is dominated by forest/grassland mosaic. While natural vegetation should be dominated by rain forest on Nosy Be, these last results indicate that human continuously impacted the landscapes surrounding the lake. At a local scale, our data support the “subsistence shift hypothesis” which proposed that population expansion with development of herding/farming altered the natural ecosystems. However, a precise regional synthesis is challenging, since high-resolution multi-proxy records from continuous sedimentary archives as well as records located further north and in the hinterland are still scarce in Madagascar. The lack of such regional synthesis also prevents precise comparison between different regions in Madagascar to detect potential (dis)similarities in climate dynamics, ecosystem responses and anthropogenic influences at the island’s scale during the (late) Holocene.

Spatial distribution and consequences of contaminants in harbour sediments - A case study from Richards Bay Harbour, South Africa

Richards Bay Harbour (RBH) is situated in the industrialized area on the northeast coast of South Africa. To decipher recent human activities and accompanying environmental degradation, surface sediment was collected across RBH and analysed for granulometric and elemental composition, microfaunal assemblages, and microplastics. Microplastics occur most abundantly near recreational areas, whereas metal contamination relates to activities at bulk goods terminals from which they are imported or exported. In particular, Cr and Cu concentrations in surface sediment near bulk goods terminals exceed South African sediment quality guidelines. In metal contaminated sediment, bioindicators reflected stress and were noticeably impacted. A transect of short sediment cores reflects spatial and historical metal contamination and allows quantification of the load of metals within the sediment column. The volume of metal (Cr) contaminated sediment was estimated at almost 2 million m³.

The potential of δ2Hn-alkanes and δ18Osugar for paleoclimate reconstruction - A regional calibration study for South Africa

The hydrogen isotopic composition of leaf wax-derived n-alkanes (δ²H_n-alkanes) is a widely applied proxy for (paleo)climatic changes. It has been suggested that the coupling with the oxygen isotopic composition of hemicellulose-derived sugars (δ¹⁸O_sugar) - an approach dubbed 'paleohygrometer' - might allow more robust and quantitative (paleo)hydrological reconstructions. However, the paleohygrometer remains to be evaluated and tested regionally. In this study, topsoil samples from South Africa, covering extensive environmental gradients, are analysed. δ²H_n-alkanes correlates significantly with the isotopic composition of precipitation (δ²H_p), whereas no significant correlation exists between δ¹⁸O_sugar and δ¹⁸O_p. The apparent fractionation (ε_app) is the difference between δ²H_n-alkanes and δ²H_p (ε_{app 2H}) and δ¹⁸O_sugar and δ¹⁸O_p (ε_{app 18O}), respectively, and integrates i) isotopic enrichment due to soil water evaporation, ii) leaf (and xylem) water transpiration and iii) biosynthetic fractionation. We find no correlation of ε_{app 18O} nor for ε_{app 2H} with temperature, and no correlation of ε_{app 2H} with potential evapotranspiration and an aridity index. By contrast, ε_{app 18O} correlates significantly with both potential evapotranspiration and the aridity index. This highlights the strong effect of evapotranspirative enrichment on δ¹⁸O_sugar. In study areas without plant predominance using Crassulacean Acid Metabolism (CAM), coupling δ¹⁸O_sugar and δ²H_n-alkanes enables to reconstruct δ²H_p and δ¹⁸O_p with an offset of Δδ²H = 6 ± 27‰ and Δδ¹⁸O = 0.8 ± 3.7‰, respectively, as well as relative humidity (RH) with an offset of ΔRH = 6 ± 17%. The paleohygrometer does, however, not work well for our study areas where CAM plants prevail (reconstructed δ¹⁸O_p, δ²H_p and RH are off by 3.1‰, 27.2‰ and 31.7%). This probably reflects plant-specific (phenological) adaptations and/or post-photosynthetic exchange reactions related to CAM metabolism. Overall, our findings corroborate that δ²H_n-alkanes and δ¹⁸O_sugar are valuable proxies, and the paleohygrometer is a promising approach for paleoclimate reconstructions in southern Africa.

Late Holocene vegetation responses to climate change and human impact on the central Tibetan Plateau

Understanding long-term environmental changes under natural and anthropic forces is helpful for facilitating sustainable development. Here we present a sedimentary record from the central Tibetan Plateau to investigate the impacts of climate and human activities on alpine vegetation during the late Holocene, based on a 162-cm-long lacustrine sediment core collected from Tangra Yumco. Palynology, charcoal and minerogenic input reveal variations of climate and human activity during the past 3400 cal yr BP. Our results show that alpine steppe dominated by Artemisia, Cyperaceae and Poaceae was present in the Tangra Yumco area during the entire covered period. Only minor human activities are visible between 3400 and 2300 as well as from 1700 to 400 cal yr BP, when vegetation was mainly influenced by climate. Although human activities (presence/grazing) became more intensive between 2300 and 1700 cal yr BP corresponding to the Zhang Zhung Kingdom, vegetation change is still mainly affected by a more arid climate. Strongest human influence on vegetation was found after 400 cal yr BP, when vegetation composition was altered by farming and grazing activities. Our records indicate human activities did not have significant impacts on alpine environment until the past few centuries at Tangra Yumco on the central Tibetan Plateau.

A persistent northern boundary of Indian Summer Monsoon precipitation over Central Asia during the Holocene

Extra-tropical circulation systems impede poleward moisture advection by the Indian Summer Monsoon. In this context, the Himalayan range is believed to insulate the south Asian circulation from extra-tropical influences and to delineate the northern extent of the Indian Summer Monsoon in central Asia. Paleoclimatic evidence, however, suggests increased moisture availability in the Early Holocene north of the Himalayan range which is attributed to an intensification of the Indian Summer Monsoon. Nevertheless, mechanisms leading to a surpassing of the Himalayan range and the northern maximum extent of summer monsoonal influence remain unknown. Here we show that the Kunlun barrier on the northern Tibetan Plateau [~36°N] delimits Indian Summer Monsoon precipitation during the Holocene. The presence of the barrier relocates the insulation effect 1,000 km further north, allowing a continental low intensity branch of the Indian Summer Monsoon which is persistent throughout the Holocene. Precipitation intensities at its northern extent seem to be driven by differentiated solar heating of the Northern Hemisphere indicating dependency on energy-gradients rather than absolute radiation intensities. The identified spatial constraints of monsoonal precipitation will facilitate the prediction of future monsoonal precipitation patterns in Central Asia under varying climatic conditions.

Climate change on the Tibetan Plateau in response to shifting atmospheric circulation since the LGM

The Tibetan Plateau (TP) is primarily influenced by the northern hemispheric middle latitude Westerlies and the Indian summer monsoon (ISM). The extent, long-distance effects and potential long-term changes of these two atmospheric circulations are not yet fully understood. Here, we analyse modern airborne pollen in a transition zone of seasonally alternating dominance of the Westerlies and the ISM to develop a pollen discrimination index (PDI) that allows us to distinguish between the intensities of the two circulation systems. This index is applied to interpret a continuous lacustrine sedimentary record from Lake Nam Co covering the past 24 cal kyr BP to investigate long-term variations in the atmospheric circulation systems. Climatic variations on the central TP widely correspond to those of the North Atlantic (NA) realm, but are controlled through different mechanisms resulting from the changing climatic conditions since the Last Glacial Maximum (LGM). During the LGM, until 16.5 cal kyr BP, the TP was dominated by the Westerlies. After 16.5 cal kyr BP, the climatic conditions were mainly controlled by the ISM. From 11.6 to 9 cal kyr BP, the TP was exposed to enhanced solar radiation at the low latitudes, resulting in greater water availability.