Aragonite pteropod flux to the Somali Basin, NW Arabian Sea (original) (raw)
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Seasonal variation of pteropods from the Western Arabian Sea sediment trap
Geo-marine Letters, 2006
Sediment trap samples collected from the Western Arabian Sea yielded a rich assemblage of intact and non-living (opaque white) pteropod tests from a water depth of 919 m during January to September 1993. Nine species of pteropods were recorded, all (except one) displaying distinct seasonality in abundance, suggesting their response to changing hydrographical conditions influenced by the summer/winter monsoon cycle. Pteropod fluxes increased during the April–May peak of the intermonsoon, and reached maximum levels in the late phase of the southwest summer monsoon, probably due to the shallowing of the mixed layer depth. This shallowing, coupled with enhanced nutrient availability, provides ideal conditions for pteropod growth, also reflected in corresponding fluctuations in the flux of the foraminifer Globigerina bulloides. Pteropod/planktic foraminifer ratios displayed marked seasonal variations, the values increasing during the warmer months of April and May when planktic foraminiferal fluxes declined. The variation in fluxes of calcium carbonate, organic carbon and biogenic opal show positive correlations with fluxes of pteropods and planktic foraminifers. Calcium carbonate was the main contributor to the total particulate flux, especially during the SW monsoon. In the study area, pteropod flux variations are similar to the other flux patterns, indicating that they, too could be used as a potential tool for palaeoclimatic reconstruction of the recent past.
The preservational record of pteropod shells (aragonite) for the last 30 kyr has been studied in a core (SK17) recovered from the eastern Arabian Sea margin at off Goa coast (depth 840m, lat. 15°15'N, long. 72°44'E). The chronostratigraphy of the core established on the basis of high resolution stable isotope record of a planktic foraminifera (Globigerinoides ruber) and several AMS radiocarbon ages demonstrates millennial scale variation in δ 18 O defining Younger Dryas and Heinrich like Events. Records of absolute abundance of pteropods (1g/dry wt >125 µm), abundance ratio of pteropod and planktic foraminifera, aragonite (wt %), organic carbon (OC) (wt %) and CaCO 3 (wt %) show major changes during these isotopic events. Aragonite maxima and higher number of well-preserved pteropod shells, are noticed during cold stadial periods. The study indicates a negative correlation between aragonite and OC % (productivity index). On the other hand, total CaCO 3 content (calcite and aragonite) is positively correlated with the aragonite. The variation patterns of pteropod shells and aragonite content in the sediment core are suggested to be controlled by the preservational conditions associated with the fluctuation in Aragonite Compensation Depth (ACD) and Oxygen Minimum Zone (OMZ) intensity. It is suggested that the high biological productivity during intensified summer monsoons in late Holocene and inter-stadial periods might have resulted in severe oxygen depletion (strong OMZ) leading to shallowing of the ACD. A weak summer monsoon and low productivity condition prevailing during cold stadial periods would have resulted in a weak OMZ and deepening of the ACD.
Earth System Science Data, 2012
Pteropods are a group of holoplanktonic gastropods for which global biomass distribution patterns remain poorly described. The aim of this study was to collect and synthesise existing pteropod (Gymnosomata, Thecosomata and Pseudothecosomata) abundance and biomass data, in order to evaluate the global distribution of pteropod carbon biomass, with a particular emphasis on temporal and spatial patterns. We collected 25 939 data points from several online databases and 41 scientific articles. These data points corresponded to observations from 15 134 stations, where 93 % of observations were of shelled pteropods (Thecosomata) and 7 % of non-shelled pteropods (Gymnosomata). The biomass data has been gridded onto a 360 × 180 • grid, with a vertical resolution of 33 depth levels. Both the raw data file and the gridded data in NetCDF format can be downloaded from PANGAEA, doi:10.1594/PANGAEA.777387. Data were collected between 1950-2010, with sampling depths ranging from 0-2000 m. Pteropod biomass data was either extracted directly or derived through converting abundance to biomass with pteropod-specific length to carbon biomass conversion algorithms. In the Northern Hemisphere (NH), the data were distributed quite evenly throughout the year, whereas sampling in the Southern Hemisphere (SH) was biased towards winter and summer values. 86 % of all biomass values were located in the NH, most (37 %) within the latitudinal band of 30-60 • N. The range of global biomass values spanned over four orders of magnitude, with mean and median (non-zero) biomass values of 4.6 mg C m −3 (SD = 62.5) and 0.015 mg C m −3 , respectively. The highest mean biomass was located in the SH within the 70-80 • S latitudinal band (39.71 mg C m −3 , SD = 93.00), while the highest median biomass was in the NH, between 40-50 • S (0.06 mg C m −3 , SD = 79.94). Shelled pteropods constituted a mean global carbonate biomass of 23.17 mg CaCO 3 m −3 (based on non-zero records). Total biomass values were lowest in the equatorial regions and equally high at both poles. Pteropods were found at least to depths of 1000 m, with the highest biomass values located in the surface layer (0-10 m) and gradually decreasing with depth, with values in excess of 100 mg C m −3 only found above 200 m depth. Tropical species tended to concentrate at greater depths than temperate or high-latitude species. Global biomass levels in the NH were relatively invariant over the seasonal cycle, but more seasonally variable in the SH. The collected database provides a valuable tool for modellers for the study of marine ecosystem processes and global biogeochemical cycles. By extrapolating regional biomass to a global scale, we established global pteropod biomass to add up to 500 Tg C.
Quaternary Science Reviews, 2007
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).
Late Quaternary millennial-scale variability in pelagic aragonite preservation off Somalia
Marine Micropaleontology, 2006
In order to better understand Late Quaternary pelagic aragonite preservation in the western Arabian Sea we have investigated a high-resolution sediment core 905 off Somalia. Pteropod preservation is enhanced in times of reduced monsoon-driven productivity, indicated by low amounts of C org and low barium to aluminium (Ba/Al) ratios. All periods corresponding to Heinrich events in the North Atlantic are represented by maxima in shell preservation of the common pteropod Limacina inflata (LDX values b 2, except for H5-equivalent with a poorer shell preservation, LDX N 2.66). Good shell preservation is also found during stadials at 52.1-53.2, 36, 33.2, and 31.9 ka. Relative abundance of pteropods and their fragments in the coarse fraction reaches maxima during Marine Isotope Stage (MIS) 5.2, during time-equivalents of Heinrich events 4-6 and in stadials at ∼ 53, ∼42.5, and 41.4 ka.
A first look at factors affecting aragonite compensation depth in the eastern Arabian Sea
Palaeogeography, Palaeoclimatology, Palaeoecology, 2017
Water column measurements suggest shoaling of aragonite saturation depths (ASD) throughout the world oceans, due to increase in greenhouse gas concentration. Past records of aragonite saturation state under different climatic conditions are required to assess the impact of climatic changes on shoaling/deepening of ASD. The preservation state of organisms having aragonite skeletons, is used to assess the past changes in aragonite saturation depths, with respect to the modern ASD. Here for the first time, we delineate and discuss the factors that affect the modern aragonite compensation depth (ACD) in the eastern Arabian Sea by using pteropod abundance in the surface sediments. A total of 78 spade core-top samples collected along seven latitudinal transects, covering the continental shelf, slope and abyssal region of the eastern Arabian Sea were used. Pteropods were picked from coarse fraction (≥63 μm). Based on the pteropod preservation, we report that in the eastern Arabian Sea, ACD lies at a water depth of ≤525 m, which matches with the chemically defined aragonite saturation depth. We further report that the ACD shoals from north to south. The zone of high pteropod abundance coincides with low %C org. The increase in pteropod abundance in the outer shelf region coincides with the drop in dissolved oxygen concentration. The deeper limit of pteropod abundance lies in the center of the oxygen minimum zone with higher %C org. Therefore, we suggest that the pteropod abundance in the eastern Arabian Sea is not always related with the lower dissolved oxygen, but is strongly influenced by %C org. This first report of the pteropod based aragonite compensation depth estimates from the eastern Arabian Sea will help in assessing future changes in ACD under the influence of anthropogenic greenhouse gas emissions.
Recent and Late Quaternary pteropod preservation on the Pakistan shelf and continental slope
Marine Geology, 2006
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.
Journal of Micropalaeontology, 2005
This paper is the first contribution to the knowledge of pteropods in surficial sediments of the entire western continental shelf of India. Both qualitative and quantitative analyses of pteropod assemblages were carried out on 50 coarse fractions (>125 µm) of topmost portions of grab samples recovered along 12 transects covering the inner shelf to the upper slope (30-200 m water depths). The distribution of eight significant species was mapped. The resultant maps were correlated with the bathymetric, hydrographic and aragonite preservation conditions in the investigated area. This study reveals a bathymetric control over the distribution of Limacina inflata, Creseis acicula and Creseis chierchiae across the shelf. Furthermore, a marked difference in vertical salinity gradient from south to north along the shelf appears to have a profound influence on the abundance of these species. The pteropod shells in sediments of the northern offshore region with anoxic bottom conditions are generally well preserved, unlike the southern region, where oxygen concentration of the bottom water is relatively high enough to cause dissolution or even elimination of many shells of the most susceptible pteropods like C. acicula and C. chierchiae. The variation in the intensity of the oxygen minimum zone along the western continental shelf of India has significant influence upon the distribution pattern of Clio convexa.