Pteropod preservation on the Pakistan shelf and continental slope

Fifteen surface sediment samples from the Pakistan shelf and upper continental slope and a Late Quaternary high-sedimentation rate core (573 m water depth, Pakistan continental margin) have been analysed to improve the understanding of the factors influencing pteropod preservation. The aragonite compensation depth (ACD) is located at 250-400 m water depth, which corroborates previous observations of a very shallow ACD in the northern Arabian Sea. With the exception of the Hab transect off Karachi, the ACD coincides with the upper boundary of the OMZ located at 250 m water depth. The shell preservation index of the pteropod Limacina inflata (LDX) was applied on six surface sediment samples showing good to very good preservation (LDX: 2.2 to 1.3). The 30 000 yr long record of sediment core SO90 137KA is characterized by alternations between bioturbated and laminated sediments. Bioturbated sediments occurring in the Early Holocene, Younger Dryas and time-equivalents of Heinrich events contain well to perfectly preserved tests of L. inflata (LDX: 2.1-0.2), whereas only traces of pteropods are found in laminated intervals. The close linkage of pteropod preservation in the surface sediments and in core 137KA to well-oxygenated conditions can be explained by repetitive intermediate water formation in the Arabian Sea down to at least 600 m water depth in times of enhanced NE monsoons during stadials and H-equivalents. Low amounts of pteropods in laminated sediments (interstadials, Late Holocene) and in the present-day oxygen minimum zone (OMZ) indicate a weak NE monsoon, stable OMZ and shallow ACD.

Siliceous fossils in sediments of the Ibero-Moroccan shelf and adjacent deep sea plains

Siliceous skeletons were investigated in two core profiles (9 cores), one off Cap de Sines, Portugal and the other off Cap de Mazagan, Morocco. Total number of skeletons was determined per gram of dried sediment at different core depths of the fraction >21 µ. Results are compared with a core profile from the Arabian Sea. Diatoms are of four groups: (A) marine-planktonic, B) marine-benthic, (C) freshwater and (D) Tertiary species (Trinacria e.g.). Species from groups (B), (C) and (D) are redeposited in all cores taken at a water depth of greater than 100 m. Small numbers of Silicoflagellates and Radiolarians were found throughout the cores from the Ibero-Moroccan shelf. In the Arabian Sea core, Radiolarians were concentrated in distinct horizons in which Tertiary material was redeposited (40-50, 140-150, 250-260 cm). The number of siliceous skeletons per gram of dried sediment decreases more or less rapidly with increasing depth in all cores. Whereas about 2500 skeletons were found in sediments close to the surface, approximately 100 skeletons only were found in deeper (>40 cm) layers. Deeper horizons with more than 100 specimens were interpreted as redeposited material. This sediment contained robust skeletons, resistant against dissolution, as well as benthic and Tertiary material. The decrease of siliceous skeletons relative to core depth depends upon the sedimentation rate. Where the sedimentation rate is high, the opal dissolution zone extends down to 30-60 cm, where the sedimentation rate is low, it is located at 10-30 cm. Below these depths opals disappears. These zones also have approximately the same age (4000 years) everywhere. Siliceous skeletons dissolve differentially, first the Silicoflagellates disappear, second the Diatoms, third the Radiolarians, and fourth the Sponge Spicules. Surface structure of skeletons from near the opal dissolution zones are similar to those of skeletons treated with NaOH. Tertiary diatoms (Trinacria e. g.) and benthic diatoms (Campylodiscus e.g.) dissolve less rapidly than skeletons of modern planktonic diatoms (Coscinodiscus e.g.). The time control of the opal dissolution zones appeared rather independent of various oceanic influences. No evidence was found for effects from upwelling either off Portugal or off Morocco. No difference in dissolution rates was recorded between the abyssal plains lying off these two areas. Likewise, there was no change in solution rates from Pleistocene to Holocene within either one of the abyssal plains. The Mediterranean outflow, which is enriched in dissolved silica, apparently had no effect on dissolution rates of siliceous skeletons in the sediment.

Heterocyst glycolipids in surface sediments and water samples of the Amazon shelf

Marine endosymbiotic heterocystous cyanobacteria make unique heterocyst glycolipids (HGs) containing pentose (C5) moieties. Functionally similar HGs with hexose (C6) moieties found in free-living cyanobacteria occur in the sedimentary record, but C5 HGs have not been documented in the natural environment. Here we developed a high performance liquid chromatography multiple reaction monitoring (MRM) mass spectrometry (HPLC-MS2) method specific for trace analysis of long chain C5HGs and applied it to cultures of Rhizosolenia clevei Ostenfeld and its symbiont Richelia intracellularis which were found to contain C5 HGs and no C6 HGs. The method was then applied to suspended particulate matter (SPM) and surface sediment from the Amazon plume region known to harbor marine diatoms carrying heterocystous cyanobacteria as endosymbionts. C5 HGs were detected in both marine SPM and surface sediments, but not in SPM or surface sediment from freshwater settings in the Amazon basin. Rather, the latter contained C6 HGs, established biomarkers for free-living heterocystous cyanobacteria. Our results indicate that the C5 HGs may be potential biomarkers for marine endosymbiotic heterocystous cyanobacteria.

Sedimentology of the Vietnam shelf

On the Vietnam Shelf more than 1000 miles of shallow high-resolution seismics were analyzed to unravel post-glacial evolution in a tropical, siliciclastic environment together with 25 sediment cores from water depths between 21 and 169 m to determine stratigraphy, distribution and style of sedimentation. Fourty-seven samples were dated with the AMS-14C technique. The shelf was grouped into three regions: a southern part, a central part, and a northern part. On the broad Southern Shelf, sedimentation is influenced by the Mekong River, which drains into the SCS in this area. Here, incised valley fills are abundant that were cut into the late Pleistocene land surface by the Paleo-Mekong River during times of sea level lowstand. Those valleys are filled with transgressive deposits. The Holocene sedimentation rate in this low gradient accommodation-dominated depositional system is in the range of 5-10 and 25-40 cm/ky at locations sheltered from currents. The Central Shelf is narrow and the sedimentary strata are conformable. Here, numerous small mountainous rivers reach the SCS and transport large amounts of detrital sediment onto the shelf. Therefore, the Holocene sedimentation rate is high with values of 50-100 cm/ky in this supply-dominated depositional system. The broad Northern Shelf in the vicinity of the Red River Delta shows, as on the Southern Shelf, incised valleys cut into the Pleistocene land surface by paleo river channels. In this accommodation-dominated shelf area, the sedimentation rate is low with values of 5-10 cm/ky. Where applicable, we assigned the sampled deposits to different paleo-facies. The latter are related to certain intervals of water depths at their time of deposition. Comparison with the sea-level curve of (Hanebuth et al., 2000, doi:10.1126/science.288.5468.1033) indicates subsidence on the Central Shelf, which is in agreement with the high sedimentation rates in this area. In contrast, data from the Northern Shelf suggest tectonic uplift that might be related to recent tectonic movements along the Ailao Shan-Red River Fault zone. Data from the Southern Shelf are generally in agreement with the sea-level curve mentioned above.

(Table 1) Characteristics of Klebsormidium (filamentous green algae) strains from the Antarctic, Arctic and Slovakia

The freezing and desiccation tolerance of 12 Klebsormidium strains, isolated from various habitats (aero-terrestrial, terrestrial, and hydro-terrestrial) from distinct geographical regions (Antarctic - South Shetlands, King George Island, Arctic - Ellesmere Island, Svalbard, Central Europe - Slovakia) were studied. Each strain was exposed to several freezing (-4°C, -40°C, -196°C) and desiccation (+4°C and +20°C) regimes, simulating both natural and semi-natural freeze-thaw and desiccation cycles. The level of resistance (or the survival capacity) was evaluated by chlorophyll a content, viability, and chlorophyll fluorescence evaluations. No statistical differences (Kruskal-Wallis tests) between strains originating from different regions were observed. All strains tested were highly resistant to both freezing and desiccation injuries. Freezing down to -196°C was the most harmful regime for all studied strains. Freezing at -4°C did not influence the survival of studied strains. Further, freezing down to -40°C (at a speed of 4°C/min) was not fatal for most of the strains. RDA analysis showed that certain Antarctic and Arctic strains did not survive desiccation at +4°C; however, freezing at -40°C, as well as desiccation at +20 °C was not fatal to them. On the other hand, other strains from the Antarctic, the Arctic, and Central Europe (Slovakia) survived desiccation at temperatures of +4°C, and freezing down to -40°C. It appears that species of Klebsormidium which occupy an environment where both seasonal and diurnal variations of water availability prevail, are well adapted to freezing and desiccation injuries. Freezing and desiccation tolerance is not species-specific nor is the resilience only found in polar strains as it is also a feature of temperate strains.

Holocene d18O diatom from ODP Hole 178-1098A

The causes for rising temperatures along the Antarctic Peninsula during the late Holocene have been debated, particularly in light of instrumental records of warming over the past decades (Russell and McGregor, 2010, doi:10.1007/s10584-009-9673-4). Suggested mechanisms range from upwelling of warm deep waters onto the continental shelf in response to variations in the westerly winds (Bentley et al., 2009, doi:10.1177/0959683608096603), to an influence of El Niño-Southern Oscillation on sea surface temperatures (Shevenell et al., 2011, doi:10.1038/nature09751). Here, we present a record of Holocene glacial ice discharge, derived from the oxygen isotope composition of marine diatoms from Palmer Deep along the west Antarctic Peninsula continental margin. We assess atmospheric versus oceanic influences on glacial discharge at this location, using analyses of diatom geochemistry to reconstruct atmospherically forced glacial ice discharge and diatom assemblage (Taylor and Sjunneskog, 2002, doi:10.1029/2000PA000564) ecology to investigate the oceanic environment. We show that two processes of atmospheric forcing-an increasing occurrence of La Niña events (Makou et al., 2010, doi:10.1130/G30366.1) and rising levels of summer insolation-had a stronger influence during the late Holocene than oceanic processes driven by southern westerly winds and upwelling of upper Circumpolar Deepwater. Given that the evolution of El Niño-Southern Oscillation under global warming is uncertain (Yeh et al., 2009, doi:10.1038/nature08316), its future impacts on the climatically sensitive system of the Antarctic Peninsula Ice Sheet remain to be established.