Sediment fingerprinting as an environmental forensics tool explaining cyanobacteria blooms in lakes

Cyanobacteria (blue-green algae) blooms in water bodies present serious public health issues with attendant economic and ecological impacts. Llyn Tegid (Lake Bala) is an important conservation and amenity asset within Snowdonia National Park, Wales which since the mid-1990s has experienced multiple toxic cyanobacteria blooms threatening the ecology and tourism-dependent local economy. Multiple working hypotheses explain the emergence of this problem, including climate change, land management linked to increased nutrient flux, hydromorphological alterations or changing trophic structure - any of which may operate individually or cumulatively to impair lake function. This paper reports the findings of a sedimentfingerprinting study using dated lake cores to explore the linkages between catchment and lake management practices and the emergence of the algal blooms problem. Since 1900 AD lake bed sedimentation rates have varied from 0.06 to 1.07 g cm−2 yr−1, with a pronounced acceleration since the early 1980s. Geochemical analysis revealed increases in the concentrations of total phosphorus (TP), calcium and heavy metals such as zinc and lead consistent with eutrophication and a rising pollution burden, particularly since the late 1970s. An uncertainty-inclusive sedimentfingerprinting approach was used to apportion the relative fluxes from the major catchment land cover types of improved pasture, rough grazing, forestry and channel banks. This showed improved pasture and channel banks are the dominant diffuse sources of sediment in the catchment, though forestry sources were important historically. Conversion of rough grazing to improved grassland, coupled with intensified land management and year-round livestock grazing, is concluded to provide the principal source of rising TP levels. Lake Habitat Survey and particle size analysis of lake cores demonstrate the hydromorphological impact of the River Dee Regulation Scheme, which controls water level and periodically diverts flow into Llyn Tegid from the adjacent Afon Tryweryn catchment. This hydromorphological impact has also been most pronounced since the late 1970s. It is concluded that an integrated approach combining land management to reduce agricultural runoff allied to improved water level regulation enabling recovery of littoral macrophytes offers the greatest chance halting the on-going cyanobacteria issue in Llyn Tegid.

Age models and stabile isotope ratios from SO35 cores

Stable isotope measurements on the planktonic foraminifer Globigerinoides ruber (white) have been carried out on a number of selected deep-seas sediment cores from the South Lau and Norlh Fiji Basins. The d18O-curves show good correlation with the inter-ocean oraphic correlation composite d18O-record of the standard reference section (Prell et al. 1986), which, in combination with the chronostratigraphic classifications of Herterich & Sarnthein (1984, modified) and Imbrie et al. 1984), allows a detailed dating of the sedimentary sequences. The deepest layers in core no. 119 (southern Lau Basin) could be assigned to Isotope Stage 24. Measurements made on bulk carbonate in two cores show a much higher glacial-interglacial amplitude, allowing the general identification of the conventional oxygen isotope stages. The d13C-values of the benthic foraminifer Cibicidoides wuellerstorfi show progressively lighter values northwards reflecting an increasing contribution of the isotopically lighter CO2 from the remineralisation of organic matter during the general northward movement of the deep water masses. Cyclicities in the sedimentation rates were observed in core nos. 117 and 119 (both southern Lau Basin) where the interglacials exhibit higher levels than the glacials. Calculated new or export paleoproductivity show that the glacials had higher productivity in the euphotic zone. From the oxygen isotope stratigraphy, the five ash layers in core nos. 117 and 119 could be dated as about 530 ka B.P. in Stage 14, 695 ka B.P. in Stage 18, 775 ka B.P. in Stage 21, 790 ka B.P. and 825 ka B.P. in Stage 22. Carbonate dissolution occurred during stages 5, 8 and 10 to 12.

Sedimentology and stable oxygen isotope ratios of foraminifera from cores of the Porcupine Seabight, Northeast Atlantic

Large carbonate mound structures have been discovered in the northern Porcupine Seabight (Northeast Atlantic) at depths between 600 and 1000 m. These mounds are associated with the growth of deep-sea corals Lophelia pertusa and Madrepra oculata. In this study, three sediment cores have been analysed. They are from locations close to Propeller Mound, a 150 m high ridge-like feature covered with a cold-water coral ecosystem at its upper flanks. The investigations are concentrated on grain-size analyses, carbon measurements and on the visual description of the cores and computer tomographic images, to evaluate sediment content and structure. The cores portray the depositional history of the past ~31 kyr BP, mainly controlled by sea-level fluctuations and the climate regime with the advance and retreat of the Irish Ice Sheet onto the Irish Mainland Shelf. A first advance of glaciers is indicated by a turbiditic release slightly older than 31 kyr BP, coherent with Heinrich event 3 deposition. During Late Marine Isotope Stage 3 (MIS 3) and MIS 2 shelf erosion prevailed with abundant gravity flows and turbidity currents. A change from glaciomarine to hemipelagic contourite sedimentation during the onset of the Holocene indicates the establishment of the strong, present-day hydrodynamic regime at intermediate depths. The general decrease in accumulation of sediments with decreasing distance towards Propeller Mound suggests that currents (turbidity currents, gravity flows, bottom currents) had a generally stronger impact on the sediment accumulation at the mound base for the past ~31 kyr BP, respectively.

Sedimentology on 19 cores from the South China Sea

Based on the study of 10 sediment cores and 40 core-top samples from the South China Sea (SCS) we obtained proxy records of past changes in East Asian monsoon climate on millennial to bidecadal time scales over the last 220,000 years. Climate proxies such as global sea level, estimates of paleotemperature, salinity, and nutrients in surface water, ventilation of deep water, paleowind strength, freshwater lids, fluvial and/or eolian sediment supply, and sediment winnowing on the sea floor were derived from planktonic and benthic stable-isotope records, the distribution of siliciclastic grain sizes, planktonic foraminifera species, and the UK37 biomarker index. Four cores were AMS-14C-dated. Two different regimes of monsoon circulation dominated the SCS over the last two glacial cycles, being linked to the minima and maxima of Northern Hemisphere solar insolation. (1) Glacial stages led to a stable estuarine circulation and a strong O2-minimum layer via a closure of the Borneo sea strait. Strong northeast monsoon and cool surface water occurred during winter, in part fed by an inflow from the north tip of Luzon. In contrast, summer temperatures were as high as during interglacials, hence the seasonality was strong. Low wetness in subtropical South China was opposed to large river input from the emerged Sunda shelf, serving as glacial refuge for tropical forest. (2) Interglacials were marked by a strong inflow of warm water via the Borneo sea strait, intense upwelling southeast of Vietnam and continental wetness in China during summer, weaker northeast monsoon and high sea-surface temperatures during winter, i.e. low seasonality. On top of the long-term variations we found millennial- to centennial-scale cold and dry, warm and humid spells during the Holocene, glacial Terminations I and II, and Stage 3. The spells were coeval with published variations in the Indian monsoon and probably, with the cold Heinrich and warm Dansgaard-Oeschger events recorded in Greenland ice cores, thus suggesting global climatic teleconnections. Holocene oscillations in the runoff from South China centered around periodicities of 775 years, ascribed to subharmonics of the 1500-year cycle in oceanic thermohaline circulation. 102/84-year cycles are tentatively assigned to the Gleissberg period of solar activity. Phase relationships among various monsoon proxies near the onset of Termination IA suggest that summer-monsoon rains and fluvial runoff from South China had already intensified right after the last glacial maximum (LGM) insolation minimum, coeval with the start of Antarctic ice melt, prior to the d18O signals of global sea-level rise. Vice versa, the strength of winter-monsoon winds decreased in short centennial steps only 3000-4000 years later, along with the melt of glacial ice sheets in the Northern Hemisphere.

Stable carbon and nitrogen isotopic ratios of cores from the Bering Sea

Stable carbon and nitrogen isotopic ratios (d13C and d15N) of organic matter were measured in three sediment cores from deep basins of the Bering Sea to investigate past changes in surface nutrient conditions. For surface water reconstructions, hemipelagic layers in the cores were distinguished from turbidite layers (on the basis of their sedimentary structures and 14C ages) and analyzed for isotopic studies. Although d13C profiles may have been affected by diagenesis, both d15N and d13C values showed common positive anomalies during the last deglaciation. We explain these anomalies as reflecting suppressed vertical mixing and low nutrient concentrations in surface waters caused by injection of meltwater from alpine glaciers around the Bering Sea.

Age determinations, dinoflagellate cyst and subpolar foraminifera measurements from sediment core HH11-134-BC, from the west Spitsbergen slope

Three marine sediment cores distributed along the Norwegian (MD95-2011), Barents Sea (JM09-KA11-GC), and Svalbard (HH11-134-BC) continental margins have been investigated in order to reconstruct changes in the poleward flow of Atlantic waters (AW) and in the nature of upper surface water masses within the eastern Nordic Seas over the last 3000 yr. These reconstructions are based on a limited set of coccolith proxies: the abundance ratio between Emiliania huxleyi and Coccolithus pelagicus, an index of Atlantic vs. Polar/Arctic surface water masses; and Gephyrocapsa muellerae, a drifted coccolith species from the temperate North Atlantic, whose abundance changes are related to variations in the strength of the North Atlantic Current. The entire investigated area, from 66 to 77° N, was affected by an overall increase in AW flow from 3000 cal yr BP (before present) to the present. The long-term modulation of westerlies' strength and location, which are essentially driven by the dominant mode of the North Atlantic Oscillation (NAO), is thought to explain the observed dynamics of poleward AW flow. The same mechanism also reconciles the recorded opposite zonal shifts in the location of the Arctic front between the area off western Norway and the western Barents Sea-eastern Fram Strait region. The Little Ice Age (LIA) was governed by deteriorating conditions, with Arctic/Polar waters dominating in the surface off western Svalbard and western Barents Sea, possibly associated with both severe sea ice conditions and a strongly reduced AW strength. A sudden short pulse of resumed high WSC (West Spitsbergen Current) flow interrupted this cold spell in eastern Fram Strait from 330 to 410 cal yr BP. Our dataset not only confirms the high amplitude warming of surface waters at the turn of the 19th century off western Svalbard, it also shows that such a warming was primarily induced by an excess flow of AW which stands as unprecedented over the last 3000 yr.