Clay mineralogy and organic carbon content of sediment core GeoB1401-4

Late Quaternary sediments recovered in a core from the area of the Zaire Fan, Central Africa, were analyzed for clay mineral composition in order to reconstruct fluctuations in the sediment input and freshwater discharge of the Zaire River. Clay mineral assemblages are dominated by kaolinite and smectite, which both originate mainly from the Zaire River and contain only minor contributions of eolian dust. Smectite crystallinity and chemical character of illites (Fe, Mg- or Al-rich) are used to track sediment input from the Zaire River and assess fluctuations in the freshwater discharge. Both parameters record a high-latitude forcing of river runoff at 100 ka periodicities reflecting glacial aridity and increased runoff during interglacials 1, 5 and 7. This signal is also observed in kaolinite/smectite ratios which represent the extension and intensity of the freshwater plume of the Zaire River. Clay mineral proxies reveal that river discharge and associated sediment input fluctuated in tune with precessional cycles of African monsoon intensity. Increased eolian input of kaolinite-rich dust with intensified northeast trades during glacials flattens the precessional signal in kaolinite/smectite ratios.

Palynology from surface sediment samples of the Arctic Ocean

Freshwater chlorophycean algae are characteristic organic-walled microfossils in recent coastal and shelf sediments from the Beaufort, Laptev and Kara seas (Arctic Ocean). The persistent occurrence of the chlorophycean algae Pediastrum spp. and Botryococcus cf. braunii in marine palynomorph assemblages is related to the discharge of freshwater and suspended matter from the large Siberian and North American rivers into the Arctic shelf seas. The distribution patterns of these algae in the marine environments reflect the predominant deposition of riverine sediments and organic matter along the salinity gradient from the outer estuaries and prodeltas to the shelf break. Sedimentary processes overprint the primary distribution of these algae. Resuspension of sediments by waves and bottom currents may transport sediments in the bottom nepheloid layer along the submarine channels to the shelf break. Bottom sediments and microfossils may be incorporated into sea ice during freeze-up in autumn and winter leading to an export from the shelves into the deep sea. The presence of these freshwater algae in sea-ice and bottom sediments in the central Arctic Ocean confirm that transport in sea ice is an important process which leads to a redistribution of shallow water microfossils.

(Table 3) Boron and chlorine in atmospheric moisture and rain water collected on the shore of the Golubaya Bay in 1970

Boron and chlorine were determined in rain water and in atmospheric moisture condensed in a "Saratov" refrigerator. Ocean is the main source of boron on the earth surface. Boron evaporates from the ocean and enriches atmospheric precipitation: B/Cl ratio of ocean water (0.00024) increases by factor of 10-15. Assuming that the average Cl content in global river runoff is 7.8 mg/l and boron content 0.013 mgl, B/Cl ratio in this runoff is 0.0017. The average B/Cl ratio in rain water of the Golubaya (Blue) Bay (Gelendzhik, Black Sea region) is 0.0026 and in condensates of atmospheric moisture during onshore and offshore winds in the same region it averages from 0.0029 to 0.0033. The maximum boron content in the condensates of this region during onshore winds was 0.032 mg/l and the minimum during offshore winds, 0.004 mg/l. /Cl ratio in sea water over the Atlantic Ocean and in the Gelendzhik area of the Black Sea varied within narrow range, mostly from 0.0025 to 0.0035. Similar B/Cl ratio (0.0024) was found for atmospheric precipitation on the slope of the Terskei Ala-Tau near the Issyk-Kul Lake in 1969. Thus, although chemistries of boron and chlorine (in chlorides) are very different, the B/Cl ratio in the atmosphere is fairly constant. This can be taken as a confirmation of an assumption that salt composition of sea water passes into the atmosphere in molecularly dispersed state. Supposing that the ocean-atmosphere system is in equilibrium as regards to the boron budget, it can be assumed that the same amount of boron passes from the ocean into bottom sediments and from lithosphere rocks and soils into the hydrosphere.

Stable isotopes and Mg/Ca ratios on foraminifera and Ba/Ca ratios of sediment core SK168/GC-1

The past variability of the South Asian Monsoon is mostly known from records of wind strength over the Arabian Sea while high-resolution paleorecords from regions of strong monsoon precipitation are still lacking. Here, we present records of past monsoon variability obtained from sediment core SK 168/GC-1, which was collected at the Alcock Seamount complex in the Andaman Sea. We utilize the ecological habitats of different planktic foraminiferal species to reconstruct freshwater-induced stratification based on paired Mg/Ca and d18O analyses and to estimate seawater d18O (d18Osw). The difference between surface and thermocline temperatures (delta T) and d18Osw (delta d18Osw) is used to investigate changes in upper ocean stratification. Additionally, Ba/Ca in G. sacculifer tests is used as a direct proxy for riverine runoff and sea surface salinity (SSS) changes related to monsoon precipitation on land. Our delta d18Osw time series reveals that upper ocean salinity stratification did not change significantly throughout the last glacial suggesting little influence of NH insolation changes. The strongest increase in temperature gradients between the mixed layer and the thermocline is recorded for the mid-Holocene and indicate the presence of a significantly shallower thermocline. In line with previous work, the d18Osw and Ba/Ca records demonstrate that monsoon climate during the LGM was characterized by a significantly weaker southwest monsoon circulation and strongly reduced runoff. Based on our data the South Asian Summer Monsoon (SAM) over the Irrawaddyy strengthened gradually after the LGM beginning at ~18 ka. This is some 3 kyrs before an increase of the Ba/Ca record from the Arabian Sea and indicates that South Asian Monsoon climate dynamics are more complex than the simple N-S displacement of the ITCZ as generally described for other regions. Minimum d18Osw values recorded during the mid-Holocene are in phase with Ba/Ca marking a stronger monsoon precipitation, which is consistent with model simulations.

Stable isotope ratios on planktonic foraminifer N. pachyderma of surface sediments from the Arctic Ocean (Table 1)

Planktic foraminifers Neogloboquadrina pachyderma (sin.) from 87 eastern and central Arctic Ocean surface sediment samples were analyzed for stable oxygen and carbon isotope composition. Additional results from 52 stations were taken from the literature. The lateral distribution of delta18O (18O/16O) values in the Arctic Ocean reveals a pattern of roughly parallel, W-E stretching zones in the Eurasian Basin, each ~0.5 per mil wide on the delta18O scale. The low horizontal and vertical temperature variability in the Arctic halocline waters (0-100 m) suggests only little influence of temperature on the oxygen isotope distribution of N. pachyderma (sin.). The zone of maximum delta18O values of up to 3.8 per mil is situated in the southern Nansen Basin and relates to the tongue of saline (> 33%.) Atlantic waters entering the Arctic Ocean through the Fram Strait. delta18O values decrease both to the Barents Shelf and to the North Pole, in accordance with the decreasing salinities of the halocline waters. In the Nansen Basin, a strong N-S delta18O gradient is in contrast with a relatively low salinity change and suggests contributions from different freshwater sources, i.e. salinity reduction from sea ice meltwater in the south and from light isotope waters (meteoric precipitation and river-runoff) in the northern part of the basin. North of the Gakkel Ridge, delta18O and salinity gradients are in good accordance and suggest less influence of sea ice melting processes. The delta13C (13C/12C) values of N. pachyderma (sin.) from Arctic Ocean surface sediment samples are generally high (0.75-0.95 per mil). Lower values in the southern Eurasian Basin appear to be related to the intrusion of Atlantic waters. The high delta13C values are evidence for well ventilated surface waters. Because the perennial Arctic sea ice cover largely prevents atmosphere-ocean gas exchange, ventilation on the seasonally open shelves must be of major importance. Lack of delta13C gradients along the main routes of the ice drift from the Siberian shelves to the Fram Strait suggests that primary production (i.e. CO2 consumption) does probably not change the CO2 budget of the Arctic Ocean significantly.

Geochemical analysis of two sediment cores from the southern Mendeleev Ridge

We present results of an inorganic geochemical pore water and sediment study conducted on Quaternary sediments from the western Arctic Ocean. The sediment cores were recovered in 2008 from the southern Mendeleev Ridge during RV Polarstern Expedition ARK-XXIII/3. With respect to sediment sources and depositional processes, peaks in Ca/Al, Mg/Al, Sr/Al and Sr/Mg indicate enhanced input of both ice-rafted (mainly dolomite) and biogenic carbonate during deglacial warming phases. Distinct and repetitive brown layers enriched in Mn (oxyhydr)oxides occur mostly in association with these carbonate-rich intervals. For the first time, we show that the brown layers are also consistently enriched in scavenged trace metals Co, Cu, Mo and Ni. The bioturbation patterns of the brown layers, specifically well-defined brown burrows into the underlying sediments, support formation close to the sediment-water interface. The Mn and trace metal enrichments were probably initiated under warmer climate conditions. Both river runoff and melting sea ice delivered trace metals to the Arctic Ocean, but also enhanced seasonal productivity and organic matter export to the sea floor. As Mn (oxyhydr)oxides and scavenged trace metals were deposited at the sea floor, a co-occurring organic matter "pulse" triggered intense diagenetic Mn cycling at the sediment-water interface. These processes resulted in the formation of Mn and trace metal enrichments, but almost complete organic matter degradation. As warmer conditions ceased, reduced riverine runoff and/or a solid sea ice cover terminated the input of riverine trace metal and fresh organic matter, and greyish-yellowish sediments poor in Mn and trace metals were deposited. Oxygen depletion of Arctic bottom waters as potential cause for the lack of Mn enrichments during glacial intervals is highly improbable. While the original composition and texture of the brown layers resulted from specific climatic conditions (including transient Mn redox cycling at the sediment-water interface), pore water data show that early diagenetic Mn redistribution is still affecting the organic-poor sediments in several meters depth. Given persistent steady state diagenetic conditions, purely authigenic Mn-rich brown layers may form, while others may completely vanish. The degree of diagenetic Mn redistribution largely depends on the depositional environment within the Arctic Ocean, the availability of Mn and organic matter, and seems to be recorded by the Co/Mo ratios of single Mn-rich layers. We conclude that brown Arctic sediment layers are not necessarily synchronous features, and correlating them across different parts of the Arctic Ocean without additional age control is not recommended.

Pollen and spore counts of sediment core GeoB12624

In tropical eastern Africa, vegetation distribution is largely controlled by regional hydrology, which has varied over the past 20 000 years. Therefore, accurate reconstructions of past vegetation and hydrological changes are crucial for a better understanding of climate variability in the tropical southeastern African region. We present high-resolution pollen records from a marine sediment core recovered offshore of the Rufiji River delta. Our data document significant shifts in pollen assemblages during the last deglaciation, identifying, through changes in both upland and lowland vegetation, specific responses of plant communities to atmospheric (precipitation) and coastal (coastal dynamics and sea-level changes) alterations. Specifically, arid conditions reflected by a maximum pollen representation of dry and open vegetation occurred during the Northern Hemisphere cold Heinrich event 1 (H1), suggesting that the expansion of drier upland vegetation was synchronous with cold Northern Hemisphere conditions. This arid period is followed by an interval in which forest and humid woodlands expanded, indicating a hydrologic shift towards more humid conditions. Droughts during H1 and the shift to humid conditions around 14.8 kyr BP in the uplands are consistent with latitudinal shifts of the intertropical convergence zone (ITCZ) driven by high-latitude Northern Hemisphere climatic fluctuations. Additionally, our results show that the lowland vegetation, consisting of well-developed salt marshes and mangroves in a successional pattern typical for vegetation occurring in intertidal habitats, has responded mainly to local coastal dynamics related to marine inundation frequencies and soil salinity in the Rufiji Delta as well as to the local moisture availability. Lowland vegetation shows a substantial expansion of mangrove trees after ~ 14.8 kyr BP, suggesting an increased moisture availability and river runoff in the coastal area. The results of this study highlight the decoupled climatic and environmental processes to which the vegetation in the uplands and the Rufiji Delta has responded during the last deglaciation.

R-ArcticNet, A Regional Hydrographic Data Network for the Pan-Arctic Region (ISO-image of CD-ROM)

We describe the contemporary hydrography of the pan-Arctic land area draining into the Arctic Ocean, northern Bering Sea, and Hudson Bay on the basis of observational records of river discharge and computed runoff. The Regional Arctic Hydrographic Network data set, R-ArcticNET, is presented, which is based on 3754 recording stations drawn from Russian, Canadian, European, and U.S. archives. R-ArcticNET represents the single largest data compendium of observed discharge in the Arctic. Approximately 73% of the nonglaciated area of the pan-Arctic is monitored by at least one river discharge gage giving a mean gage density of 168 gages per 106 km2. Average annual runoff is 212 mm yr?1 with approximately 60% of the river discharge occurring from April to July. Gridded runoff surfaces are generated for the gaged portion of the pan-Arctic region to investigate global change signals. Siberia and Alaska showed increases in winter runoff during the 1980s relative to the 1960s and 1970s during annual and seasonal periods. These changes are consistent with observations of change in the climatology of the region. Western Canada experienced decreased spring and summer runoff.

Results of modelled Arctic river discharge trends since 7 ka BP

The Arctic hydrological cycle throughout the Holocene is analyzed based on the results of transient simulations with the coupled atmosphere-ocean circulation model ECHO-G. The results suggest a ~ 2 % increase of mid-Holocene to preindustrial Arctic river discharges for the Eurasian continent. However, rivers of the North America Arctic realm show a moderate runoff decline of approximately 4 to 5 % for the same period. The total river discharge into the Arctic Ocean has remained at an approximately constant preindustrial level since the mid Holocene. The positive discharge trend within Eurasia is caused by a more rapid decrease in local net evaporation compared to a smaller decline in advected moisture and hence precipitation. This effect is neither recognized within the North American Arctic domain nor in the far eastern part of the Eurasian Arctic realm. A detailed comparison of these model findings with a variety of proxy studies is conducted. The collected proxy records show trends of continental surface temperatures and precipitation rates that are consistent with the simulations. A continuation of the transient Holocene runs for the 19th and 20th century with increased greenhouse gases indicates an increase of the total river influx into the Arctic Ocean of up to 7.6 %. The Eurasian river discharges increase by 7.5 %, the North American discharges by up to 8.4 %. The most rapid increases have been detected since the beginning of the 20th century. These results are corroborated by the observed rising of Arctic river discharges during the last century which is attributed to anthropogenic warming. The acceleration of the Arctic hydrological cycle in the 20th century is without precedence in the Holocene.