Relative abundance of euphausiids (Crustacea) in water samples of the Arabian Sea (Table 2)

In the present paper, the ecology and feeding habits of euphausiids are described. The samples were taken at the time of the NE-monsoon (1964/65) by R. V. "Meteor" in the Arabian Sea and adjacent waters. 24 species were determined. According to distribution of the species, the following marine areas can be distinguished: Arabian Sea: 24 species, dominant are Euphausia diomedeae, E. tenera, E. distinguenda, Stylocheiron carinatum. Gulf of Aden: 10 species, dominant are Euphausia diomedeae, E. distinguenda. Red Sea: 6 species, dominant are Euphausia diomedeae, E. distinguenda. Gulf of Oman : 5 Species, dominant are Euphausia distinguenda, Pseudeupbaufia latifrons. Persian Gulf: 1 species - Pseudeuphausia latifrons. The total number of euphausiids indicate the biomass of this group. High densities of euphausiids (200-299 and > 300 individuals/100 m**3) occur in the innermost part of the Gulf cf Aden, in the area south of the equator near the African east coast, near Karachi (Indian west coast) and in the Persian Gulf. Comparison with data relating to production biology confirms that these are eutrophic zones which coincide with areas in which upwelling occurs at the time of the NE-monsoon. The central part of the Arabian Sea differs from adjacent waters by virtue of less dense euphausiid populations (> 199 individuals/100 m**3). Measurements relating to production biology demonstrate a relatively low concentration of primary food sources. Food material was ascertained by analysis of stomach content. The following omnivorous species were examined: Euphausia diomedeae, E. distinguenda, E. tenera, Pseudeuphausia latifrons and Thysanopoda tricuspidata. Apart from crustacean remains large numbers of Foraminifera, Radiolaria, tintinnids, dinoflagellates were found in the stomachs. Quantitatively crustaceans form the most important item in the diet. Food selection on the basis of size and form appears to be restricted to certain genera of tintinnids. The genera Stylocheiron and Nematoscelis are predators. Only crustacean remains were found in the stomachs of Stylocheiron abbreviatum, whereas Radiolaria, Foraminifera and tintinnids occurred to some extent in Nematasceli sp. Different euphausiids in the food chain in the Arabian Sea. In omnivorous species the position is variable, since they not only feed by filtering autotrophic and heterotrophic Protista, but also by predation on zooplankton. Carnivorous species without filtering apparatus feed exclusively on zooplankton of the size of copepods. Only these species are well established as occupying a higher position in the food chain. The parasitic protozoan Tbalassomyces fagei was found on Euphausia diomedeae, E. fenera, E. distinguenda and E. sanzoi.

Distribution of diatoms, coccolithophores and planktic foraminifera in the Arabian Sea

The distribution of diatoms, coccolithophores and planktic foraminifers mirrored the hydrographic and trophic conditions of the surface ocean (0-100 m) across the upwelling area off the Oman coast to the central Arabian Sea during May/June 1997 and July/August 1995. The number of diatoms was increased in waters with local temperature minimum and enhanced nutrient concentration (nitrate, phosphate, silicate) caused by upwelling. Vegetative cells of Chaetoceros dominated the diatom assemblage in the coastal upwelling area. Towards the more nutrient depleted and stratified surface waters to the southeast, the number of diatoms decreased, coccolithophore and planktic foraminiferal numbers increased, and floral and faunal composition changed. In particular, the transition between the eutrophic upwelling region off Oman and the oligotrophic central Arabian Sea was marked by moderate nutrient concentration, and high coccolithophore and foraminifer numbers. Florisphaera profunda, previously often referred as a 'lower-photic-zone-species', was frequent in water depths as shallow as 20 m, and at high nutrient concentration up to 14 µmol NO3/l and 1.2 µmol PO4/. To the oligotrophic southeast of the divergence, cell densities of coccolithophores declined and Umbellosphaera irregularis prevailed throughout the water column down to 100 m depth. In general, total coccolithophore numbers were limited by nutrient threshold concentration, with low numbers (<10*10**3 cells/l) at high [NO3] and [PO4], and high numbers (>70*10**3 cells/l) at low [NO3] and [PO4]. The components of the complex microplankton succession, diatoms, coccoliths and planktic foraminifers (and possibly others), should ideally be used as a combined paleoceanographic proxy. Consequently, models on plankton ecology should be resolved at least for the seasonality, to account for the bias of paleoceanographic transfer calculations.

Stable oxygen and carbon isotope ratios of the planktonic foraminifera Pulleniatina obliquiloculata and the benthic foraminifera Uvigerina excellens of ODP Site 117-723 in the Arabian Sea (Table 1)

Site 723 is located in a water depth of 808 m at the center of the oxygen minimum zone and the middle part of the main thermocline on the Oman Margin. Oxygen isotope curves of planktonic delta18OP and benthic delta18OB can be traced back continuously to Stage 23 with high resolution measurements. A tentative correlation to Stage 53 has been tried using oxygen isotope stratigraphy. The amplitudes of the fluctuations of the benthic delta18OB curve are small, compared with the planktonic delta18OP curve. The delays of benthic oxygen isotopes delta18OB related to the planktonic delta18OP appear in the transgressive stages. Carbon isotopes of benthic delta13CB and planktonic delta13CP generally show an inverse correlation with oxygen isotope values delta18OB and delta18OB and delta18OP, however, the changes of delta13C are more gradual than those of delta18O during transgressive stages in spite of the synchronized changes of delta13C with those of delta18O during regressive stages. The difference of oxygen isotope between benthic and planktonic foraminifers represents the degree of pushing up the thermocline by upwelling, and the difference of carbon isotope represents the relative amount of upwelling Sigma[CO2] to the biological uptake in the surface water. These isotopic differences can be used as indicators of upwelling and show strong upwelling in the interglacial and weak upwelling in the glacial stages. The organic carbon content is correlated with the isotopic upwelling indicators, and higher content is correlated with the isotopic upwelling indicators and higher content appears in the interglacial stages. The calculated rate of sedimentation based on oxygen isotope stratigraphy in glacial stages is significantly high, two to four times that of interglacial stages, and the absolute flux of fluvial sediments with variability of lithofacies increased in the glacial stage. The present glacial-interglacial cycle with the fluctuation of upwelling relating to the southwest monsoon can be traced back to Stage 8, 250 ka. From Stage 8 to 12, 250-450 ka, the upwelling indicator of oxygen isotope difference did not show such distinct cyclicity. For Stages 12-15, 450-600 ka, the upwelling can be estimated as strong as in interglacial stage of the present cycles, with slightly weak upwelling in the glacial stage. This upwelling and climate can be traced back to the late Pliocene. The strongest upwelling can be estimated in the Pliocene-Pleistocene time by the isotopic indicators and the high organic carbon content.

Coccolithophores in surface sediments of the Arabian Sea (TAble 1)

One hundred surface sediment samples of the Arabian Sea (Indian Ocean) were investigated and relative abundances of coccoliths were compared to mean annual gradients of temperature, salinity, chlorophyll, PO4 and mixed layer depth. Total coccolith concentrations ranged from 42*10**6/g sediment in coastal areas to more than 19000*10**6/g sediment in oceanic regions. The general distribution does not seem to be dependent on coccolithophore productivity in surface waters alone, but also on the diluting input of terrigenous material. A total of 27 taxa were identified. The main species dominating the assemblages were Gephyrocapsa oceanica, Emiliania huxleyi and Florisphaera profunda with a combined average abundance of more than 70%. Several species and species groups reflect with their distribution the environmental parameters of the overlying water masses and may be successfully used to improve palaeoclimatic reconstructions, e.g. (a) F. profunda exhibits a high similarity or even positive correlation to the mean annual mixed layer depth, (b) calciosolenids can be described as coastal or shelf species. While temperature and salinity gradients do not seem to be crucial for coccolithophores in this region, the mean mixed layer depth as well as the PO4 concentration (representative for total nutrient availability) may control in part the coccolithophore assemblages. According to the results of a cluster analysis and the distribution pattern of all species, it was possible to differentiate three main coccolithophore assemblages. A G. oceanica dominated assemblage mainly occurs in the northern part of the study area and can be described as 'high nutrient assemblage'. The second assemblage, dominated by F. profunda, may be typical for oligotrophic and stable conditions in open ocean waters. A third assemblage, with high amounts of 'coastal species', characterises coastal conditions on the shelves.

Sea surface fugacity of carbon dioxide measurements in the Pacific ocean

As part of the JGOFS field program, extensive CO2 partial-pressure measurements were made in the atmosphere and in the surface waters of the equatorial Pacific from 1992 to 1999. For the first time, we are able to determine how processes occurring in the western portion of the equatorial Pacific impact the sea-air fluxes of CO2 in the central and eastern regions. These 8 years of data are compared with the decade of the 1980s. Over this period, surface-water pCO2 data indicate significant seasonal and interannual variations. The largest decreases in fluxes were associated with the 1991-94 and 1997-98 El Niño events. The lower sea-air CO2 fluxes during these two El Niño periods were the result of the combined effects of interconnected large-scale and locally forced physical processes: (1) development of a low-salinity surface cap as part of the formation of the warm pool in the western and central equatorial Pacific, (2) deepening of the thermocline by propagating Kelvin waves in the eastern Pacific, and (3) the weakening of the winds in the eastern half of the basin. These processes serve to reduce pCO2 values in the central and eastern equatorial Pacific towards near-equilibrium values at the height of the warm phase of ENSO. In the western equatorial Pacific there is a small but significant increase in seawater pCO2 during strong El Niño events (i.e., 1982-83 and 1997-98) and little or no change during weak El Niño events (1991-94). The net effect of these interannual variations is a lower-than-normal CO2 flux to the atmosphere from the equatorial Pacific during El Niño. The annual average fluxes indicate that during strong El Niños the release to the atmosphere is 0.2-0.4 Pg C/yr compared to 0.8-1.0 Pg C/yr during non-El Niño years.

AMS 14C dating points, isotopic compositions, Mg/Ca ratios and sea surface temperatures of sediment cores GeoB10029-4 and GeoB10038-4

Quantifying the spatial and temporal sea surface temperature (SST) and salinity changes of the Indo-Pacific Warm Pool is essential to understand the role of this region in connection with abrupt climate changes particularly during the last deglaciation. In this study we reconstruct SST and seawater d18O of the tropical eastern Indian Ocean for the past 40,000 years from two sediment cores (GeoB 10029-4, 1°30'S, 100°08'E, and GeoB 10038-4, 5°56'S, 103°15'E) retrieved offshore Sumatra. Our results show that annual mean SSTs increased about 2-3 °C at 19,000 years ago and exhibited southern hemisphere-like timing and pattern during the last deglaciation. Our SST records together with other Mg/Ca-based SST reconstructions around Indonesia do not track the monsoon variation since the last glacial period, as recorded by terrestrial monsoon archives. However, the spatial SST heterogeneity might be a result of changing monsoon intensity that shifts either the annual mean SSTs or the seasonality of G. ruber towards the warmer or the cooler season at different locations. Seawater d18O reconstructions north of the equator suggest fresher surface conditions during the last glacial and track the northern high-latitude climate change during the last deglaciation. In contrast, seawater ?18O records south of the equator do not show a significant difference between the last glacial period and the Holocene, and lack Bølling-Allerød and Younger Dryas periods suggestive of additional controls on annual mean surface hydrology in this part of the Indo-Pacific Warm Pool.

Strontium measurements and sea surface temperatures of corals from Xisha Island, South China Sea

A Porites coral collected from Xisha Island, South China Sea, represents a skeleton secreted in the period from 1906 to 1994. The Sr contents of the coral vary linearly with the instrument-measured sea-surface temperature (SST), giving a Sr thermometer: SST = -1.9658 x Sr + 193.26. The reconstructed SST data show that the late 20th century was warmer (about 1°C) than the early 20th century and that two cooling (1915/1916 and 1947/1948) and three warming (1935/1936, 1960/1961, and 1976/1977) shifts occurred in the century. The temperature shifts are more pronounced for winters, implying a close effect of the west Pacific warm pool and Asian monsoon and suggesting that the former is a primary force controlling the climatic system of the region. Results of this study and previously published data indicate a close link of temperature shifts between the boreal summer and the austral winter or the boreal winter and the austral summer. The annual SST anomalies in the South China Sea and the South Pacific reveal the existence of harmonic but opposite SST variations between the two regions. On the decadal scale the comparative annual SST anomalies for the South China Sea and for the equatorial west Pacific show a similarity in temperature variations, implying that the South China Sea climate is coherent with climatic regime of the tropical west Pacific.

Stable isotope record, Mg/Ca ratios and sea surface reconstruction for sediment core MD06-3067

We present sea surface, upper thermocline, and benthic d18O data, as well as temperature and paleoproductivity proxy data, from the International Marine Global Change Study Program (IMAGES) Core MD06-3067 (6°31'N, 126°30'E, 1575 m water depth), located in the western equatorial Pacific Ocean within the flow path of the Mindanao Current. Our records reveal considerable glacial-interglacial and suborbital variability in the Mindanao Dome upwelling over the last 160 kyr. Dome activity generally intensified during glacial intervals resulting in cooler thermocline waters, whereas it substantially declined during interglacials, in particular in the early Holocene and early marine oxygen isotope stage (MIS) 5e, when upwelling waters did not reach the thermocline. During MIS 3 and MIS 2, enhanced surface productivity together with remarkably low SST and low upper ocean thermal contrast provide evidence for episodic glacial upwelling to the surface, whereas transient surface warming marks periodic collapses of the Mindanao Dome upwelling during Heinrich events. We attribute the high variability during MIS 3 and MIS 2 to changes in the El Niño Southern Oscillation state that affected boreal winter monsoonal winds and upper ocean circulation. Glacial upwelling intensified when a strong cyclonic gyre became established, whereas El Niño-like conditions during Heinrich events tended to suppress the cyclonic circulation, reducing Ekman transport. Thus, our findings demonstrate that variations in the Mindanao Dome upwelling are closely linked to the position and intensity of the tropical convection and also reflect far-field influences from the high latitudes.

Sea-surface paleotemperature reconstructions for the middle Miocene-Holocene of ODP Site 133-811 in the Coral Sea off northeast Australia

The derivation of a detailed sea-surface paleotemperature curve for the middle Miocene-Holocene (10-0 Ma) from ODP Site 811 on the Queensland Plateau, northeast Australia, has clarified the role of sea-surface temperature fluctuations as a control on the initiation and development of the extensive carbonate platforms of this region. This curve was derived from isotopic analyses of the planktonic foraminifer Globigerinoides ruber, and converted to temperature using the surface-water paleotemperature equation accounting for variations in global ice volume. The accuracy of these data were confirmed by derivation of paleotemperatures using the water column isotopic gradient (Delta delta18O), corrected for salinity and variations in seafloor water mass temperature. Results indicate that during this period surface-water temperatures were, on average, greater than the minimum required for tropical reef growth (20°C; Veron, 1986), with the exception of the late Miocene and earliest early Pliocene (10-4.9 Ma), when there were repeated intervals of temperatures between 18-20°C. Tropical reef growth on the Queensland Plateau was extensive from the early to early middle Miocene (~21-13 Ma), after which reef development began to decline. A lowstand near 11 Ma probably exposed shallower portions of the plateau; after re-immersion near 7 Ma, the areal extent of reef development was greatly reduced (~ 50%). Paleotemperature data from Site 811 indicate that decreased sea-surface temperatures were likely to have been instrumental in reducing the area of active reef growth on the Queensland Plateau. Reduced reefal growth rates continued until the late Pliocene or Quaternary, despite the increase of average sea-surface paleotemperatures to 22-23°C. Studies on modern corals show that when sea-surface temperatures are below ~24°C, as they were from the late Miocene to the Pleistocene off northeast Australia, corals are stressed and growth rates are greatly reduced. Consequently, when temperatures are in this range, corals have difficulty keeping pace with subsidence and changing environmental factors. In the late Pliocene, sedimentation rates increased due to increases in non-reefal carbonate production and falling sea levels. It was not until the mid-Quaternary (0.6-0.7 Ma) that sea-surface paleotemperatures increased above 24°C as a result of the formation of a western Coral Sea warm water pool. Because of age discrepancies, it is unclear exactly when an effective barrier developed on the central Great Barrier Reef; the formation of the warm water pool was likely to have either assisted the formation of this barrier and/or permitted increased coral growth rates. Fluctuations in sea-surface temperature can account for much of the observed spatial and temporal variations of reef growth and carbonate platform distribution off northeast Australia, and therefore we conclude that paleotemperature variations are a critical control on the development of carbonate platforms, and must be considered an important cause of ancient platform "drowning".