Dynamics of the last four glacial terminations recorded in Lake Van, Turkey

A well-dated suite of Lake Van climate-proxy data covering the last 360 ka documents environmental changes over 4 glacial/interglacial cycles in Eastern Anatolia, Turkey. The picture of cold and dry glacials and warm and wet interglacials emerging from pollen, organic carbon, authigenic carbonate content, elemental profiling by XRF and lithological analyses is inconsistent with classical interpretation of ox- ygen isotopic composition of carbonates pointing to a more complex pattern in Lake Van region. Detailed analysis of glacial terminations allows for the constraining of a depositional model explaining different patterns observed in all the proxies. We hypothesize that variations in relative contribution of rainfall, snowmelt and glacier meltwater recharging the basin have a very important role for all sedimentary processes in Lake Van. Lake level of glacial Lake Van, predominantly fed by snowmelt, was low, the water column was oxic, and carbonates precipitating in the epilimnion recorded the light isotopic signature of inflow. During terminations, increasing rainfall and significant supply of mountain glaciers' meltwater contributed to lake level rise. Increased rainfall enhanced density gradients in the water column, and hindered mixing leading to development of bottom-water anoxia. Carbonates precipitating during terminations show large fluctuations in their isotopic composition. Full interglacial conditions in Lake Van are characterized by high or slowly falling lake level. Rainfall and snowmelt feed the lake but due to re-established mixing, the isotopic composition of authigenic carbonates is heavier and closer to that of evaporation-influenced lake water than that of runoff representing snowmelt and atmospheric precipitation.

Coupled human and natural system dynamics as key to the sustainability of Lake Victoria's ecosystem services

East Africa’s Lake Victoria provides resources and services to millions of people on the lake’s shores and abroad. In particular, the lake’s fisheries are an important source of protein, employment, and international economic connections for the whole region. Nonetheless, stock dynamics are poorly understood and currently unpredictable. Furthermore, fishery dynamics are intricately connected to other supporting services of the lake as well as to lakeshore societies and economies. Much research has been carried out piecemeal on different aspects of Lake Victoria’s system; e.g., societies, biodiversity, fisheries, and eutrophication. However, to disentangle drivers and dynamics of change in this complex system, we need to put these pieces together and analyze the system as a whole. We did so by first building a qualitative model of the lake’s social-ecological system. We then investigated the model system through a qualitative loop analysis, and finally examined effects of changes on the system state and structure. The model and its contextual analysis allowed us to investigate system-wide chain reactions resulting from disturbances. Importantly, we built a tool that can be used to analyze the cascading effects of management options and establish the requirements for their success. We found that high connectedness of the system at the exploitation level, through fisheries having multiple target stocks, can increase the stocks’ vulnerability to exploitation but reduce society’s vulnerability to variability in individual stocks. We describe how there are multiple pathways to any change in the system, which makes it difficult to identify the root cause of changes but also broadens the management toolkit. Also, we illustrate how nutrient enrichment is not a self-regulating process, and that explicit management is necessary to halt or reverse eutrophication. This model is simple and usable to assess system-wide effects of management policies, and can serve as a paving stone for future quantitative analyses of system dynamics at local scales.

Long-term man–environment interactions in the Bolivian Amazon: 8000 years of vegetation dynamics

Only few studies documenting the vegetation history of the Llanos de Moxos, one of the largest seasonally flooded wetland areas in South America, are available and little is known about the environmental impact of pre-Columbian settlements. We use radiocarbon-dated terrestrial plant macrofossils to establish a sound chronology and palynological analyses to reconstruct the vegetation and fire history of the Lago Rogaguado area. The sedimentary pollen and spore record suggests that wetland and wooded savannah (Cerrado) environments occurred around the lake between 8100 and 5800 cal BP. Fire activity was high during this period and was probably connected to the dry Cerrado environments. The pollen evidence suggests early plant cultivation (Zea mays, Annonaceae and Cucurbitaceae) from 6500 cal BP onwards, which is significantly earlier than hitherto assumed for Amazonia. Gallery forests expanded after 5800 cal BP, when fire activity strongly declined. Forest expansion intensified around 2800 cal BP and continued until 2000 cal BP, when forest cover reached its maximum and fire activity its minimum. The late-Holocene forest expansion to the south and the decrease of fire activity may have resulted from a climatic shift to moister conditions (possibly a shorter dry season). New crops (e.g. Avena-type) or adventive plants (e.g. Rumex acetosella-type) document the impact of European economies after ca. 500 cal BP. Land use intensity remained rather stable over the most recent centuries, arguing against a collapse of settlements in response to the arrival of Europeans, as reconstructed from other Amazonian pollen records.

Deep water provenance and dynamics of the (de)glacial Atlantic meridional overturning circulation

Reconstructing past modes of ocean circulation is an essential task in paleoclimatology and paleoceanography. To this end, we combine two sedimentary proxies, Nd isotopes (εNd) and the 231Pa/230Th ratio, both of which are not directly involved in the global carbon cycle, but allow the reconstruction of water mass provenance and provide information about the past strength of overturning circulation, respectively. In this study, combined 231Pa/230Th and εNd down-core profiles from six Atlantic Ocean sediment cores are presented. The data set is complemented by the two available combined data sets from the literature. From this we derive a comprehensive picture of spatial and temporal patterns and the dynamic changes of the Atlantic Meridional Overturning Circulation over the past ∼25 ka. Our results provide evidence for a consistent pattern of glacial/stadial advances of Southern Sourced Water along with a northward circulation mode for all cores in the deeper (>3000 m) Atlantic. Results from shallower core sites support an active overturning cell of shoaled Northern Sourced Water during the LGM and the subsequent deglaciation. Furthermore, we report evidence for a short-lived period of intensified AMOC in the early Holocene.

Late-glacial and Holocene vegetation history and dynamics as shown by pollen and plant macrofossil analyses in annually laminated sediments from Soppensee, central Switzerland

The palynostratigraphy of two sediment cores from Soppensee, Central Switzerland (596 m asl) was correlated with nine regional pollen assemblage zones defined for the Swiss Plateau. This biostratigraphy shows that the sedimentary record of Soppensee includes the last 15 000 years, i.e. the entire Late-glacial and Holocene environmental history. The vegetation history of the Soppensee catchment was inferred by pollen and plant-macrofossil analyses on three different cores taken in the deepest part of the lake basin (27 m). On the basis of a high-resolution varve and calibrated radiocarbonchronology it was possible to estimate pollen accumulation rates, which together with the pollen percentage data, formed the basis for the interpretation of the past vegetation dynamics. The basal sediment dates back to the last glacial. After reforestation with juniper and birch at ca. 12 700 B.P., the vegetation changed at around 12 000 B.P. to a pine-birch woodland and at the onset of the Holocene to a mixed deciduous forest. At ca. 7000 B.P., fir expanded and dominated the vegetation with beech becoming predominant at ca. 50014C-years later until sometime during the Iron Age. Large-scale deforestation, especially during the Middle Ages, altered the vegetation cover drastically. During the Late-glacial period two distinct regressive phases in vegetation development are demonstrated, namely, the Aegelsee oscillation (equivalent to the Older Dryas biozone) and the Younger Dryas biozone. No unambiguous evidence for Holocene climatic change was detected at Soppensee. Human presence is indicated by early cereal pollen and distinct pulses of forest clearance as a result of human activity can be observed from the Neolithic period onwards.

Ocean dynamics, not dust, have controlled equatorial Pacific productivity over the past 500,000 years

Biological productivity in the equatorial Pacific is relatively high compared with other low-latitude regimes, especially east of the dateline, where divergence driven by the trade winds brings nutrient rich waters of the Equatorial Undercurrent to the surface. The equatorial Pacific is one of the three principal high-nutrient low chlorophyll ocean regimes where biological utilization of nitrate and phosphate is limited, in part, by the availability of iron. Throughout most of the equatorial Pacific, upwelling of water from the Equatorial Undercurrent supplies far more dissolved iron than is delivered by dust, by as much as two orders of magnitude. Nevertheless, recent studies have inferred that the greater supply of dust during ice ages stimulated greater utilization of nutrients within the region of upwelling on the equator, thereby contributing to the sequestration of carbon in the ocean interior. Here we present proxy records for dust and for biological productivity over the past 500 ky at three sites spanning the breadth of the equatorial Pacific Ocean to test the dust fertilization hypothesis. Dust supply peaked under glacial conditions, consistent with previous studies, whereas proxies of export production exhibit maxima during ice age terminations. Temporal decoupling between dust supply and biological productivity indicates that other factors, likely involving ocean dynamics, played a greater role than dust in regulating equatorial Pacific productivity.

First results of SONNE cruise SO121

The Red Sea is a very young ocean, and is one of the most interesting areas on Earth (ocean in statu nascendi). It is the only ocean where hydrothermal activity associated with ore formation occurs in a sterile environment (anoxic, hot, saline). In addition, its geographical position means that it is predestined to record the monsoonal history of the region in detailed sedimentary sequences. The major aim of the present project is to investigate the dynamics of hydrothermal systems in selected Deeps (Atlantis-II, Discovery, Kebrit, Al Wajh), Additional palaeoceanographic and microbiological questions should also be addressed. Specific aims are: 1. To study the hydrographic changes in individual Deeps (hydrothermal region Atlantis-II) and to investigate the causes of the temperature increase in the last few years (increased heat flow - higher temperature of the brine supply - higher brine flow rates?). 2.a. To document the influence of the hydrothermal systems on the sedimentary organic matter in the Deeps. In particular, the thermogenic production and migration of hydrocarbons in the sediments will be studied. The complex formation mechanisms (bacterial, thermogenic) of short-chain hydrocarbons (trace gases) will also be examined, 2.b. in addition, the polar and macromolecular fraction in samples from the various deeps will be studied in order to elucidate the formation, structure and source of the macromolecular oil fraction. 3. To clarify the palaeoceanographic conditions, sea-level changes and the climatic history (relationship of the circulation system and nutrient supply to the monsoon) of the southern Red Sea. 4. To separate microorganisms from the brines and to characterise them in terms of their metabolic physiology and ecology, and to describe their taxonomy.

Diatom studies in surface and downcore sediments off southern Sumatra and Java

A wealth of sedimentary records aimed at reconstructing late Quaternary changes in productivity and temperature have been devoted to understanding linkages between the Indo-Pacific Warm Pool (IPWP) and other distant oceanic areas. Most of these reconstructions are based, however, on biogeochemical and sedimentological proxies, with comparatively less attention devoted to microfossils. A high-resolution (<1 ka) study of diatom concentrations and the community at site GeoB10038-4, recovered off southern Sumatra (ca. 6°S, 103°E), closely tracks the variations of diatom concentrations in the westernmost IPWP during the last glacial-interglacial cycle. The diatom record provides evidence that diatom paleoproductivity was highest during interglacials, primarily due to the input of lithogenics and nutrients following the rise in sea level after full glacials. In addition, the co-variation of total diatom concentration and Northern Hemisphere forcing for Marine Isotope Stage 5 suggests a direct response of diatom productivity and upwelling intensity to boreal summer insolation. Temporal shifts of the diverse diatom community at site GeoB10038-4 correspond well with the present-day seasonal monsoon pattern and the strengthening and weakening phases of upwelling along the southern coast of Sumatra. Resting spores of Chaetoceros, typical of nutrient-rich waters, were dominant during periods of highest diatom paleoproductivity and responded to the strengthening of the SE monsoon, while diatoms of oligotrophic to mesotrophic waters characterized intermonsoon periods. The close correspondence between the dominance of upwelling diatoms and the boreal summer insolation resembles the present-day dynamics of diatom production. The observed interglacial highs and glacial lows of diatom productivity at site GeoB10038-4 is a unique pattern in the late Quaternary tropics.

Seasonal changes in labile organic matter composition on the continental shelf and bathyal sediments of the Cretan Sea

Bacterial abundance, biomass and cell size were studied in the oligotrophic sediments of the Cretan Sea (Eastern Mediterranean), in order to investigate their response to the seasonal varying organic matter (OM) inputs. Sediment samples were collected on a seasonal basis along a transect of seven stations (ranging from 40 to 1570 m depth) using a multiple-corer. Bacterial parameters were related to changes in chloroplastic pigment equivalents (CPE), the biochemical composition (proteins, lipids, carbohydrates) of the sedimentary organic matter and the OM flux measured at a fixed station over the deep basin (1570 m depth). The sediments of the Cretan Sea represent a nutrient depleted ecosystem characterised by a poor quality organic matter. All sedimentary organic compounds were found to vary seasonally, and changes were more evident on the continental shelf than in deeper sediments. Bacterial abundance and biomass in the sediments of the Cretan Sea (ranging from 1.02 to 4.59 * 10**8 cells/g equivalent to 8.7 and 38.7 µgC/g) were quite high and their distribution appeared to be closely related to the input of fresh organic material. Bacterial abundance and biomass were sensitive to changes in nutrient availability, which also controls the average cell size and the frequency of dividing cells. Bacterial abundance increased up to 3-fold between August '94 and February '95 in response to the increased amount of sedimentary proteins and CPE, indicating that benthic bacteria were constrained more by changes in quality rather than the quantity of the sedimentary organic material. Bacterial responses to the food inputs were clearly detectable down to 10 cm depth. The distribution of labile organic compounds in the sediments appeared to influence the vertical patterns of bacterial abundance and biomass. Cell size decreased significantly with water depth. Bacterial abundance and biomass were characterised by clear seasonal changes in response to seasonal OM pulses. The strong coupling between protein flux and bacterial biomass together with the strong bacterial dominance over the total biomass suggest that the major part of the carbon flow was channelled through the bacteria and the benthic microbial loop.

Controls of sediment dynamics on the Galician slope reconstructed from three sediment cores (GeoB11035-1, GeoB130206-1, GeoB13071-1)

Controls of sediment dynamics at the Galician continental slope (NW Iberia) during the past 30 ka were reconstructed from three new gravity cores (GeoB11035-1, 130206-1, 13071-1) based on sedimentological (e.g. sortable silt, IRD), micropalaeontological (e.g. coccoliths), geochemical (AMS 14C, XRF) and geophysical (e.g. magnetic susceptibility) diagnostics. The data are consistent with existing regional knowledge that, during marine isotope stages 3-1, variations in detrital input, marine productivity and sea level were the essential drivers of sediment availability on the slope, whereas deep-water current velocities controlled sediment deposition: (1) the period prior to 30 cal ka BP is characterized by minor but systematic variations in various proxies which can be associated with D-O cycles; (2) between 30 and 18 cal ka BP, high detrital input and steady slope-parallel currents led to constant sedimentation; (3) from the LGM until 10 cal ka BP, the shelf-transgressive sea-level rise increased the detrital particle flux; sedimentation was influenced by significantly enhanced deep-water circulation during the Bølling/Allerød, and subsequent slowing during the Younger Dryas; (4) an abrupt and lasting change to hemipelagic sedimentation at ca. 10 cal ka BP was probably due to Holocene warming and decelerated transgression; (5) after 5 cal ka BP, additional input of detrital material to the slope is plausibly linked to the evolution of fine-grained depocentres on the Galician shelf, this being the first report of this close shelf-slope sedimentary linkage off NW Iberia. Furthermore, there is novel evidence of the nowadays strong outer shelf Iberian Poleward Current becoming established at about 15.5 cal ka BP. The data also demonstrate that small-scale morphologic features and local pathways of sediment export from the neighbouring shelf play an important role for sediment distribution on the NW Iberian slope, including a hitherto unknown sediment conduit off the Ría de Arousa. By implication, the impact of local morphology on along- and down-slope sediment dynamics is more complex than commonly considered, and deserves future attention.

Nitrogen isotope gradients off Peru and Ecuador related to upwelling, productivity, nutrient uptake and oxygen deficiency

We present new nitrogen isotope data from the water column and surface sediments for paleo-proxy validation collected along the Peruvian and Ecuadorian margins between 1°N and 18°S. Productivity proxies in the bulk sediment (organic carbon, total nitrogen, biogenic opal, C37 alkenone concentrations) and 15N/14N ratios were measured at more than 80 locations within and outside the present-day Peruvian oxygen minimum zone (OMZ). Microbial N-loss to N2 in subsurface waters under O2 deficient conditions leaves a characteristic 15N-enriched signal in underlying sediments. We find that phytoplankton nutrient uptake in surface waters within the high nutrient, low chlorophyll (HNLC) regions of the Peruvian upwelling system influences the sedimentary signal as well. How the d15Nsed signal is linked to these processes is studied by comparing core-top values to the 15N/14N of nitrate and nitrite (d15N[NOx]) in the upper 200 m of the water column. Between 1°N and 10°S, subsurface O2 is still high enough to suppress N-loss keeping d15NNOx values relatively low in the subsurface waters. However d15N[NOx] values increase toward the surface due to partial nitrate utilization in the photic zone in this HNLC portion of the system. d15N[sed] is consistently lower than the isotopic signature of upwelled [NO3]-, likely due to the corresponding production of 15N depleted organic matter. Between 10°S and 15°S, the current position of perennial upwelling cells, HNLC conditions are relaxed and biological production and near-surface phytoplankton uptake of upwelled [NO3]- are most intense. In addition, subsurface O2 concentration decreases to levels sufficient for N-loss by denitrification and/or anammox, resulting in elevated subsurface d15N[NOx] values in the source waters for coastal upwelling. Increasingly higher production southward is reflected by various productivity proxies in the sediments, while the north-south gradient towards stronger surface [NO3]- utilization and subsurface N-loss is reflected in the surface sediment 15N/14N ratios. South of 10°S, d15N[sed] is lower than maximum water column d15N[NOx] values most likely because only a portion of the upwelled water originates from the depths where highest d15N[NOx] values prevail. Though the enrichment of d15N[NOx] in the subsurface waters is unambiguously reflected in d15N[sed] values, the magnitude of d15N[sed] enrichment depends on both the depth of upwelled waters and high subsurface d15N[NOx] values produce by N-loss. Overall, the degree of N-loss influencing subsurface d15N[NOx] values, the depth origin of upwelled waters, and the degree of near-surface nitrate utilization under HNLC conditions should be considered for the interpretation of paleo d15N[sed] records from the Peruvian oxygen minimum zone.

Coccolith fluxes in the Santa Barbara Basin

We investigate the long-term stability of El Niño-Southern Oscillation and Pacific Decadal Oscillation based on the examination of coccolithophore assemblages in a largely laminated 35 ka sedimentary record, retrieved in the Santa Barbara Basin (core MD02-2503). At a centennial scale coccolith assemblages indicate low primary production in the basin from 35 to 11.5 ka B.P., whereas the Holocene is characterized by high-productivity conditions. This pattern demonstrates the influence of the glacial-interglacial cycles on productivity and, by inference, on the nutrient supply by the upwelling cell off Point of Conception. On a shorter scale, laminations associated with Dansgaard-Oeschger events appear to be due to an injection of poorly oxygenated waters in the deepest part of the basin rather than anoxia due to high primary production. A seasonal sampling in seven laminated sections (spanning from 20 to 220 years) extracted from Holocene, Bølling-Allerød, and Dansgaard-Oeschger event 3 indicates El Niño probably existed continuously during the last 28 ka. The frequency of El Niño varied through time (between 1/2.5 and 1/5 event/a) and appearing to follow the precession cycle. El Niño exhibits higher (lower) frequencies when the precession values are lower (higher). Finally, the Holocene is characterized by a decrease in El Niño's frequencies due to the reinforcement of El Niño through this period.