Geochemical analysis of surface sediments along the Portuguese Margin

Twenty-nine surface samples from the Portuguese shelf, recovered offshore from the mouths of the Ave, Douro, Lis and Mira rivers, were analysed using ICP-OES for selected major and trace elements, after total dissolution. Organic carbon, carbonate content and grain size were also determined. Five evaluation tools have been applied in order to compare the three study areas and to evaluate sediment geochemistry and other sediment compositional variability in the acquired samples: (1) empirical methods based on comparison with standard reference criteria, e.g. the NOAA sediment quality guidelines, (2) normalisation ratios using a grain-size proxy element, (3) "Gradient Method", plotting contaminant vs. organic matter or Al, (4) definition of a regional geochemical baseline from a compiled database, and (5) enrichment factors. The evaluation of element and component associations indicates differences related both to the onshore drainage areas and to the environmental shelf setting. Despite the considerable variability in total metal contents indicated by our results, the sediment metal composition is largely of natural origin. Metal enrichments observed in the Mira area are associated with the drainage of mineralised areas rich in Cu, Pb, Zn, Fe and Mn. The near absence of human impact on shelf sediments, despite the vicinity to urban areas with high industrialisation levels, such as the Ave-Douro and Lis areas, is attributed to effective trapping in the estuaries and coastal zones, as well dilution with less contaminated sediments shelf sediments and removal with fine fractions due to grain-size sorting. The character of the contaminated sediments transported to these shelf areas is further influenced by grain-size sorting as well as by dilution with less contaminated marine sediments. The results obtained individually by the different methods complement each other and allow more specific interpretations.

Geologic-hydrologic setting and surface sedimentology in the Persian Gulf

1. Morphology and sedimentation The deepest parts of the Persian Gulf lie off the Iranian coast. Several swells separate the Persian Gulf into the Western Basin, the Central Basin and the Strait of Hormuz, which leads without noticeable morphological interruption onto the Biaban Shelf; the latter gradually drops off towards the continental slope, which itself has a strongly subdivided morphology. The sediment distribution in the Western Basin runs parallel to the basin's axis to a depth of 50 -60 m. This is caused by the shallow and uniform slope of the Iranian coast into the Western Basin, by clear exposure of the area to the Shamal-Winds and by tidal currents parallel to the basin's axis. Most other parameters also show isolines parallel to the coast line. Data from the sediment analyses show a net transport which extends out along the Central Swell: coarse fraction > 63 µ, total carbonate content, carbonate in fine fractions < 2 µ, 2-6 µ and 20-63 µ, calcite-aragonite ratios in the fine fractions 2-6 µ and 20-63 µ and quartz-dolomite ratios in fine fraction 2-6 µ. At least the uppermost 10-40 m of this sediment is late Holocene. This implies sedimentation rates of several meters per 1000 years. The slope from the Iranian coast into the Central Basin (max. depth 100 m) is generally steeper, with interspersed islands and flats. Both facts tend to disturb a sediment dustribition parallel to the basin's axis over extensive areas and may preclude any such trend from being detected by the methods and sample net used. The spatial distribution of the coarse fraction, however, seems to indicate sediment transport at greater water depths perpendicular to the basin's long axis and along the steepest gradients well into the Central Basin. The flats of the Central Basin have a sediment cover distinctly different from those of the deeper basin areas. Characteristic parameters are the extremely high percentages of coarse grained sediments, total content of carbonate CO2 over 40, low total organic carbon content, (however values are high if calculated on the basis of the < 63 µ fraction), low total N-content, and low C/N ratios. These characteristics probably result from the absence of any terrigenous material being brought in as well as from exposure to wave action. Finest terrigenous material is deposited in the innermost protected part of the Hormuz Bay. In the deep channel cut into the Biaban Shelf which carries the Persian Gulf out-flow water to the Indian Ocean, no fine grained sediment is deposited as shown by grain size data. 2. Geographic settings and sedimentation Flat lands border the Arabian coast of the Persian Gulf except for the Oman region. The high and steep Zagros Mountains form the Iranian coastline. Flat topography in combination with generally low precipitation precludes fluviatile sediment being added from the South. Inorganic and biogenic carbonates accumulating under low sedimentation rates are dominant on the shallow Arabic Shelf and the slopes into the Western and Central Basins. The fluviatile sediment brought in from the Iranian side, however decisively determine the composition of the Holocene sediment cover in the Persian Gulf and on the Biaban Shelf. Holocene sediments extend 20-30 km seaward into the Western Basin and about 25 km on to the Biaban Shelf. As mentioned before, sedimentation rates are of several meters/1000 years. The rocks exposed in the hinterland influence the sediments. According to our data the Redbeds of the Zagros Mountains determine the colour of the very fine grained sediments near the Iranian Coast of the Persian Gulf. To the West of Hormuz, addition of carbonate minerals is particularly high. Dolomite and protodolomite, deposited only in this area, as well as palygorskite, have proven to be excellent trace minerals. To the East of Hormuz, the supply of terrigenous carbonates is considerably lower. Clay minerals appear to bring in inorganically bound nitrogen thus lowering the C/N ratio in these sediments especially off river mouths. 3. Climate and sedimentation The Persian Gulf is located in a climatically arid region. This directly affects sedimentation through increased wind action and the infrequent but heavy rainfalls which cause flash floods. Such flash floods could be responsible for transporting sedheats into the Central Basin in a direction perpendicular to the Gulf's axis. Eolian influx is difficult to asses from our data; however, it probably is of minor importance from the Iranian side and may add, at the most, a few centimeters of fine sediment per 1000 years. 4. Hydrology and sedimentation High water temperatures favor inorganic carbonate precipitation in southern margin of the Gulf, and probably on the flats, as well as biogenic carbonate production in general. High evaporation plus low water inflow through rivers and precipitation cause a circulation pattern that is typical for epicontinental seas within the arid climate region. Surface water flows in from the adjoining ocean, in this case the Indian Ocean and sinks to the bottom of the Persian Gulf mainly in the northern part of the Western Basin, on the "Mesopotamischer Flachschelf" ard probably in the area of the "Arabischer Flachschelf". This sinking water continually rejuvenates the bottom out-flow water. The inflowing surface water from the Indian Ocean brings organic matter into the Persian Gulf, additional nutrients are added by the "fresh" upwelling waters of the Gulf of Oman. Both nutrients and organic matter diminish very rapidly as the water moves into the Persian Gulf. This depletion of nutrients and organic matter is the reasonfor generally low organic carbon contents of the Persian Gulf sediments. The Central Swell represents a distinct boundary, to the west of which the organic carbon content are lower than to the east when sediment samples of similar grain size distribution are compared. The outflow carries well oxygenated water over the bottom of the Persian Gulf and the resulting oxidation further decreases the content of organic matter. In the Masandam-Channel and in the Biaban-Shelf channel, the outflowing water prevents deposition of fine material and transports sediment particles well beyond the shelf margin. The outflowing water remains at a depth of 200-300 m depending on its density and releases ist suspending sediment load to the ocean floor, irrespectative of the bottom morphology. This is reflected in several parameters in which the sediments from beneath the outflow differ from nearby sediments not affected by the outflowing water. High carbonate content of total samples and of the individual size fraction as well as high aragonite and dolomite contents of individual size fractions characterize the sediment beneath the outflowing water. The tidal currents, which avt more or less parallel to the Gulf's axis, favor mixing of the water masses, they rework sediments at velocities reported here. This fact enlarges to a certain degree the extent of our interfaces which are based on only a few sample points (Persian Gulf and Biaban Shelf one sample per 620 km**2, continental slope one sample per 1000 km**2). The water on the continental slope shows and oxygen minimum at 200-1200 m which favors preservation of organically-bound carbon in the sediment. The low pH-values may even permit dissolution of carbonate minerals.

Paleo-sea surface temperature calculations in the equatorial east Atlantic

We present two ~270 kyr paleo-sea surface temperature (SST) records from the Equatorial Divergence and the South Equatorial Current derived from Mg/Ca ratios in the planktic foraminifer Globigerinoides sacculifer. The present study suggests that the magnesium signature of G. sacculifer provides a seasonal SST estimate from the upper ~50 m of the water column generated during upwelling in austral low-latitude fall/winter. Common to both down-core records is a glacial-interglacial amplitude of ~3°-3.5°C for the last climatic changes and lower Holocene and glacial oxygen isotope stage 2 temperatures compared with interglacial stage 5.5 and glacial stage 6 temperatures, respectively. The comparison to published SST estimates from alkenones, oxygen isotopes, and foraminiferal transfer function from the same core material pinpoints discrepancies and conformities between methods.

Nd and Sr isotope compositions, REE concentrations and Al/Ca ratios of different phases of surface sediments in the South Pacific

The radiogenic isotope composition of neodymium (Nd) and strontium (Sr) are useful tools to investigate present and past oceanic circulation or input of terrigenous material. We present Nd and Sr isotope compositions extracted from different sedimentary phases, including early diagenetic Fe-Mn coatings, "unclean" foraminiferal shells, fossil fish teeth, and detritus of marine surface sediments (core-tops) covering the entire midlatitude South Pacific. Comparison of detrital Nd isotope compositions to deep water values from the same locations suggests that "boundary exchange" has little influence on the Nd isotope composition of western South Pacific seawater. Concentrations of Rare Earth Elements (REE) and Al/Ca ratios of "unclean" planktonic foraminifera suggest that this phase is a reliable recorder of seawater Nd isotope composition. The signatures obtained from fish teeth and "nondecarbonated" leachates of bulk sediment Fe-Mn oxyhydroxide coatings also agree with "unclean" foraminifera. Direct comparison of Nd isotope compositions extracted using these methods with seawater Nd isotope compositions is complicated by the low accumulation rates yielding radiocarbon ages of up to 24 kyr, thus mixing the signal of different ocean circulation modes. This suggests that different past seawater Nd isotope compositions have been integrated in authigenic sediments from regions with low sedimentation rates. Combined detrital Nd and Sr isotope signatures indicate a dominant role of the Westerly winds transporting lithogenic material from South New Zealand and Southeastern Australia to the open South Pacific. The proportion of this material decreases toward the east, where supply from the Andes increases and contributions from Antarctica cannot be ruled out.

Calculated sea surface temperatures of two sediment cores

The reconstruction of ocean history employs a large variety of methods with origins in the biological, chemical, and physical sciences, and uses modern statistical techniques for the interpretation of extensive and complex data sets. Various sediment properties deliver useful information for reconstructing environmental parameters. Those properties that have a close relationship to environmental parameters are called ''proxy variables'' (''proxies'' for short). Proxies are measurable descriptors for desired (but unobservable) variables. Surface water temperature is probably the most important parameter for describing the conditions of past oceans and is crucial for climate modelling. Proxies for temperature are: abundance of microfossils dwelling in surface waters, oxygen isotope composition of planktic foraminifers, the ratio of magnesium or strontium to calcium in calcareous shells or the ratio of certain organic molecules (e.g. alkenones produced by coccolithophorids). Surface water salinity, which is important in modelling of ocean circulation, is much more difficult to reconstruct. At present there is no established method for a direct determination of this parameter. Measurements associated with the paleochemistry of bottom waters to reconstruct bottom water age and flow are made on benthic foraminifers, ostracodes, and deep-sea corals. Important geochemical tracers are d13C and Cd/Ca ratios. When using benthic foraminifers, knowledge of the sediment depth habitat of species is crucial. Reconstructions of productivity patterns are of great interest because of important links to current patterns, mixing of water masses, wind, the global carbon cycle, and biogeography. Productivity is reflected in the flux of carbon into the sediment. There are a number of fluxes other than those of organic carbon that can be useful in assessing productivity fluctuations. Among others, carbonate and opal flux have been used, as well as particulate barite. Furthermore, microfossil assemblages contain clues to the intensity of production as some species occur preferentially in high-productivity regions while others avoid these. One marker for the fertility of sub-surface waters (that is, nutrient availability) is the carbon isotope ratio within that water (13C/12C, expressed as d13C). Carbon isotope ratios in today's ocean are negatively correlated with nitrate and phosphate contents. Another tracer of phosphate content in ocean waters is the Cd/Ca ratio. The correlation between this ratio and phosphate concentrations is quite well documented. A rather new development to obtain clues on ocean fertility (nitrate utilization) is the analysis of the 15N/14N ratio in organic matter. The fractionation dynamics are analogous to those of carbon isotopes. These various ratios are captured within the organisms growing within the tagged water. A number of reconstructions of the partial pressure of CO2 have been attempted using d13C differences between planktic and benthic foraminifers and d13C values of bulk organic material or individual organic components. To define the carbon system in sea water, two elements of the system have to be known in addition to temperature. These can be any combination of total CO2 , alkalinity, or pH. To reconstruct pH, the boron isotope composition of carbonates has been used. Ba patterns have been used to infer the distribution of alkalinity in past oceans. Information relating to atmospheric circulationand climate is transported to the ocean by wind or rivers, in the form of minerals or as plant andanimal remains. The most useful tracers in this respect are silt-sized particles and pollen.

Short- versus long-term responses to changing CO2 in a coastal dinoflagellate bloom

Increasing pCO2 (partial pressure of CO2 ) in an "acidified" ocean will affect phytoplankton community structure, but manipulation experiments with assemblages briefly acclimated to simulated future conditions may not accurately predict the long-term evolutionary shifts that could affect inter-specific competitive success. We assessed community structure changes in a natural mixed dinoflagellate bloom incubated at three pCO2 levels (230, 433, and 765 ppm) in a short-term experiment (2 weeks). The four dominant species were then isolated from each treatment into clonal cultures, and maintained at all three pCO2 levels for approximately 1 year. Periodically (4, 8, and 12 months), these pCO2 -conditioned clones were recombined into artificial communities, and allowed to compete at their conditioning pCO2 level or at higher and lower levels. The dominant species in these artificial communities of CO2 -conditioned clones differed from those in the original short-term experiment, but individual species relative abundance trends across pCO2 treatments were often similar. Specific growth rates showed no strong evidence for fitness increases attributable to conditioning pCO2 level. Although pCO2 significantly structured our experimental communities, conditioning time and biotic interactions like mixotrophy also had major roles in determining competitive outcomes. New methods of carrying out extended mixed species experiments are needed to accurately predict future long-term phytoplankton community responses to changing pCO2 .

Properties of seawater (pH total scale) from a Satlantic SeaFET instrument mounted on the continuous surface water sampling system during the Tara Oceans expedition 2009-2013

The Tara Oceans Expedition (2009-2013) sampled the world oceans on board a 36 m long schooner, collecting environmental data and organisms from viruses to planktonic metazoans for later analyses using modern sequencing and state-of-the-art imaging technologies. Tara Oceans Data are particularly suited to study the genetic, morphological and functional diversity of plankton. The present data set provides continuous pH measurements made during 2013 expedition with a Satlantic SeaFET instrument that was connected to the flowthrough system. Data calibration was performed according to Bresnahan et al. (2014) (using spectrophotometric pH measurements on discrete samples (Clayton and Byrne 1993). pH_internal values were taken to calibrate the data (rather than pH_external) because of the better calibration coefficient (there was no trend associated with it). The equations of Clayton and Byrne (1993) was used to compute pH from the measured absorbance values at the temperature of measurement. The data was converted to in situ temperature using the "CO2-sys" program which can be downloaded from http://cdiac.ornl.gov/ftp/co2sys/.