Mineralogical assemblage in sediment core GeoB12309-5 and in area samples from the Makran subduction zone

In order to study late Holocene changes in sediment supply into the northern Arabian Sea, a 5.3 m long gravity core was investigated by high-resolution geochemical and mineralogical techniques. The sediment core was recovered at a water depth of 956 m from the continental slope off Pakistan and covers a time span of 5 kyr. During the late Holocene source areas delivering material to the sampling site did, however, not change and were active throughout the year.

Fossil pollen record of composite sediment core TMD from Tso Moriri, analysis of modern surface pollen samples, mean annual precipitation reconstruction, and digitisation and recalibration of different discussed palaeoclimate proxy records

This paper presents a new fossil pollen record from Tso Moriri (32°54'N, 78°19'E, 4512 m a.s.l.) and seeks to reconstruct changes in mean annual precipitation (MAP) during the last 12,000 years. This high-alpine lake occupies an area of 140 km**2 in a glacial-tectonic valley in the northwestern Himalaya. The region has a cold climate, with a MAP <300 mm, and open vegetation. The hydrology is controlled by the Indian Summer Monsoon (ISM), but winter westerly-associated precipitation also affects the regional water balance. Results indicate that precipitation levels varied significantly during the Holocene. After a rapid increase in MAP, a phase of maximum humidity was reached between ca. 11 to 9.6 cal ka BP, followed by a gradual decline in MAP. This trend parallels the reduction in the Northern Hemisphere summer insolation. Comparison of different palaeoclimate proxy records reveal evidence for a stronger Holocene decrease in precipitation in the northern versus the southern parts of the ISM domain. The long-term trend of ISM weakening is overlaid with several short periods of greater dryness, which are broadly synchronous with the North Atlantic cold spells, suggesting reduced amounts of westerly-associated winter precipitation. Compared to the mid and late Holocene, it appears that westerlies had a greater influence on the western parts of the ISM domain during the early Holocene. During this period, the westerly-associated summer precipitation belt was positioned at Mediterranean latitudes and amplified the ISM-derived precipitation. The Tso Moriri pollen record and moisture reconstructions also suggest that changes in climatic conditions affected the ancient Harappan Civilisation, which flourished in the greater Indus Valley from approximately 5.2 to 3 cal ka BP. The prolonged Holocene trend towards aridity, punctuated by an interval of increased dryness (between ca. 4.5 to 4.3 cal ka BP), may have pushed the Mature Harappan urban settlements (between ca. 4.5 to 3.9 cal ka BP) to develop more efficient agricultural practices to deal with the increasingly acute water shortages. The amplified aridity associated with North Atlantic cooling between ca. 4 to 3.6 and around 3.2 cal ka BP further hindered local agriculture, possibly causing the deurbanisation that occurred from ca. 3.9 cal ka BP and eventual collapse of the Harappan Civilisation between ca. 3.5 to 3 cal ka BP.

Planktonic foraminiferal d18O values and apparent calcification depths of core-top samples along an east-west transect across the tropical Atlantic/Caribbean

D18O values of nine tropical-subtropical planktonic foraminiferal species with different preferential habitat depths collected from 62 core-top samples along an east-west transect across the tropical Atlantic/Caribbean were used to test the applicability of interspecific d18O gradients for reconstructions of tropical upper ocean stratification. In general, the d18O difference (Delta d18O) between intermediate- and shallow-dwelling species decreases, and Delta d18O between deep and intermediate dwellers increases with increasing thermocline depth towards the west. The statistical significance of regional differences in Delta d18O highlights Delta d18O between the intermediate dwellers (in particular Globorotalia scitula and Globorotalia tumida) and the shallow dweller Globigerinoides ruber pink, as well as Delta d18O between the deep dwellers Globorotalia crassaformis or Globorotalia truncatulinoides dextral and intermediate dwellers as most sensitive to changes in tropical Atlantic thermocline depth. Based on the observed regional variations in interspecific Delta d18O, we propose a multispecies stratification index "STRAtrop" = (d18Ointermediate - d18Oshallow) / (d18Odeep - d18Oshallow) for the tropical ocean. Statistically significant differences in STRAtrop values between the E-Atlantic and the Caribbean suggest that this index may be a useful tool to monitor variations in tropical upper ocean stratification in the geological record.

Low-temperature magnetic remanence and hysteresis measurements on sediment core GeoB4901-8 from the Niger deep-sea fan

Low-temperature (LT) magnetic remanence and hysteresis measurements, in the range 300-5 K, were combined with energy dispersive spectroscopy (EDS) in order to characterize the magnetic inventory of strongly diagenetically altered sediments originating from the Niger deep-sea fan. We demonstrate the possibility of distinguishing between different compositions of members of the magnetite-ulvöspinel and ilmenite-hematite solid solution series on a set of five representative samples, two from the upper suboxic and three from the lower sulfidic anoxic zone of gravity core GeoB 4901. Highly sensitive LT magnetic measurements were performed on magnetic extracts resulting in large differences in the magnetic behavior between samples from the different layers. This emphasizes that both Fe-Ti oxide phases occur in different proportions in the two geochemical environments. Most prominent are variations in the coercivity sensitive parameter coercive field (BC). At room-temperature (RT) hysteresis loops for all extracts are narrow and yield low coercivities (6-13 mT). With decreasing temperature the loops become more pronounced and wider. At 5 K an approximately 5-fold increase in BC for the suboxic samples contrasts a 20-25-fold increase for the samples from the anoxic zone. We demonstrate that this distinct increase in BC at LT corresponds to the increasing proportion of the Ti-rich hemoilmenite phase, while Fe-rich (titano-)magnetite dominates the magnetic signal at RT. This trend is also seen in the room-temperature saturation isothermal remanent magnetization (RT-SIRM) cycles: suboxic samples show remanence curves dominated by Fe-rich mineral phases while anoxic samples display curves clearly dominated by Ti-rich particles. We show that the EDS intensity ratios of the characteristic Fe Kalpha and Ti Kalpha lines of the Fe-Ti oxides may be used to differentiate between members of the magnetite-ulvöspinel and ilmenite-hematite solid solution series. Furthermore it is possible to calculate an approximate composition for each grain if the intensity ratios of natural particles are linked to well-known standards. Thus, element spectra with high Fe/Ti intensity ratios were found to be rather typical of titanomagnetite while low Fe/Ti ratios are indicative of hemoilmenite. The EDS analyses confirm the LT magnetic results, Fe-rich magnetic phases dominate in the upper suboxic environment whereas Ti-rich magnetic phases comprise the majority of particles in the lower anoxic domain: The mineral assemblage of the upper suboxic environments is composed of magnetite (~19%), titanomagnetite (~62%), hemoilmenite (~17%) and ~2% other particles. In the lower anoxic sediments, reductive diagenetic alteration has resulted in more extensive depletion of the (titano-)magnetite phase, resulting in a relative enrichment of the hemoilmenite phase (~66%). In these strongly anoxic sediments stoichiometric magnetite is barely preserved and only ~5% titanomagnetite was detected. The remaining ~28% comprises Ti-rich particles such as pseudobrookite or rutile.