productivity collapses in the arabian sea during glacial cold phases 1 productivity in the arabian sea is one of the highest in the world it is controlled by seasonally reversing monsoonal wind driven upwelling of nutrient rich deeper waters which fuel phytoplankton growth the detailed history of upwelling induced productivity in the eastern arabian sea is unknown here we present paleoproductivity records from a composite sediment core at the millennial scale during the last 80 kyr b p these records are based on relative abundance counts of planktonic foraminifera and organic carbon contents which are shown to mainly vary in concert the eastern arabian sea upwelling induced productivity was higher in the glacial period than in the holocene but it fell repeatedly on millennial timescales these productivity declines occurred during cold events in the north atlantic region with the most pronounced changes prevailing during the heinrich events hence seasonal monsoon winds that drive upwelling induced productivity in the east were weak when the north atlantic was cold these weak winds resulted in stratification of the water column comparable to today s arabian sea stratification in the intermonsoonal period combining the new eastern with published western arabian sea results shows that the entire biological factory was severely diminished during the north atlantic heinrich events and the seasonal productivity change in the arabian sea monsoon system was reduced with year round low productivity

observational evidence for westward propagation of temperature inversions in the southeastern arabian sea 1 a warm pool forms in the southeastern arabian sea seas prior to the onset of the summer monsoon over india in early june the core of this warm pool is in the lakshadweep sea ls xbt and surface salinity data collected in the ls during may 2002 april 2003 show that temperature inversions occur off the southwest coast of india in early december with the arrival of low salinity waters from the bay of bengal the low salinity waters and the inversions propagate westward along with the downwelling rossby waves that constitute the lakshadweep sea level high inversions occur in the western ls 73 e about 40 days after they occur near the coast in the eastern ls 75 5 e they disappear in april when the tropical convergence zone moves over the seas and the warm pool engulfs the region ocean dynamics and air sea fluxes are together responsible for the formation and westward propagation of the inversions

carbonate and carbon fluctuations in the eastern arabian sea over 140 ka implications on productivity changes biological productivity in the western arabian sea was higher during interglacial than glacial times in the eastern arabian sea productivity was higher during the glacials compared to interglacials which is in sharp contrast to the southwest monsoon intensity variations to examine temporal changes in productivity in the eastern arabian sea over the last 140 ka oxygen isotopes calcium carbonate and organic carbon on three cores sl 1 4 and sk 129 cr05 were analyzed oxygen isotope records display distinct glacial and interglacial transitions in the northeastern core sl 1 and eastern arabian sea core sl 4 both calcium carbonate and organic carbon variations show no significant systematic relationship with glacial and interglacials periods in the southeastern arabian sea core sk 129 cr05 calcium carbonate shows high and low values during interglacial and glacials respectively and temporal changes in organic carbon concentration are significant only during mis 5 differential variation of calcium carbonate and organic carbon concentration at the northeastern and southeastern arabian sea and between glacials and interglacials are attributed to regional differences in sedimentation rates dilution and preservation which modify the signal of carbonate and carbon production

why is the bay of bengal less productive during summer monsoon compared to the arabian sea 1 the bay of bengal is traditionally considered to be a less productive basin compared to the arabian sea we explore the reasons for this in the central bay during summer copious rainfall and river water freshen the upper layers of the bay by 3 7 psu during summer and sst was warmer by 1 5 2 c than in the central arabian sea this leads to a strongly stratified surface layer the weaker winds over the bay are unable to erode the strongly stratified surface layer thereby restricting the turbulent wind driven vertical mixing to a shallow depth of 20 m this inhibits introduction of nutrients from below situated close to the mixed layer bottom into the upper layers while advection of nutrients rich water into the euphotic zone makes the arabian sea highly productive this process is unlikely in the bay of bengal

last glacial interglacial productivity and associated changes in the eastern arabian sea abstract we reconstruct paleo productivity and bottom water oxygenation changes during the past 32 ka from the southeastern arabian sea using absolute abundance of planktic foraminifera relative abundance of globigerina bulloides angular asymmetrical benthic foraminifera aabf measurements of total organic carbon c org caco 3 and globigerinoides ruber 18 o and 13 c the faunal and geochemical proxies suggest that productivity in the southeastern arabian sea was high during mis 3 a distinct decrease in productivity is inferred during the last glacial maximum 19 23 ka lgm bottom water was well oxygenated during mis3 only to become oxygen depleted during the lgm productivity decreased abruptly during heinrich stadial 1 hs 1 but the response to heinrich stadial 2 hs 2 was different low productivity during the early deglaciation is also synchronous with an increase in ice volume corrected 18 o 18 o sw ivc a salinity proxy between 18 9 18 3 18 9 ka bp and 15 9 15 0 16 3 ka bp and a concomitant decrease in 13 c between 18 2 17 5 18 7 ka bp and 16 4 15 3 16 7 ka bp these patterns suggest that the global 13 c minimum event during the last deglaciation was associated with the weak monsoon interval during hs 1 and a drop in productivity in tropical regions a progressive increase in productivity is observed throughout the holocene with a distinct jump at 5 4 3 8 6 3 ka bp we infer that although productivity was higher in the eastern arabian sea during most of the last glacial interval the overall resultant carbon sequestration was confined only to a restricted zone and not large enough to substantially alter atmospheric co 2

the cause of carbon isotope minimum events on glacial terminations the occurrence of carbon isotope minima at the beginning of glacial terminations is a common feature of planktic foraminifera carbon isotopic records from the indo pacific sub antarctic and south atlantic we use the 13 c record of a thermocline dwelling foraminifera neogloboquadrina dutertrei and surface temperature estimates from the eastern equatorial pacific to demonstrate that the onset of 13 c minimum events and the initiation of southern ocean warming occurred simultaneously timing agreement between the marine record and the 13 c minimum in an antarctic atmospheric record suggests that the deglacial events were a response to the breakdown of surface water stratification renewed circumpolar deep water upwelling and advection of low 13 c waters to the convergence zone at the sub antarctic front on the basis of age agreement between the absolute 13 c minimum in surface records and the shift from low to high 13 c in the deep south atlantic we suggest that the 13 c rise that marks the end of the carbon isotope minima was due to the resumption of north atlantic deep water influence in the southern ocean

fluxes of planktic foraminifera in response to monsoonal upwelling on the somalia basin margin abstract sediment trap samples collected off somalia bi weekly from early june 1992 through mid february 1993 show large seasonal variations in the shell flux and species composition of planktic foraminifera these variations mirror the monsoon driven circulation which resulted in massive upwelling and offshore eddy transport in summer during the sw monsoon june september and accounted for nearly 90 of the estimated annual shell flux of planktic foraminifera g bulloides dominated the sw monsoon along with g glutinata t quinqueloba n dutertrei and t iota in a fauna rich species but with a low diversity and equitability during the autumn intermonsoon november december the ocean became stratified and the nutrient exhausted whereas during the ne monsoon january march a warm surface mixed layer developed with the nutrients entrained by deep wind mixing it was dominated by g ruber in association with g tenella g aequilateralis g trilobus g sacculifer and g menardii in a fauna poorer in species but with a higher diversity and equitability consequently the ratio in the abundance of g bulloides and g ruber follows the monsoonal cycle closely as both species persist throughout the year only the rare g theyeri seems to be restricted to the sw monsoon in october at the end of the sw monsoon the trap at 265 m above the bottom intercepted massive amounts of fine grained carbonate m resuspended from the shelf and upper slope it contained extreme fluxes of bioclastic fragments benthic foraminifera and benthic ostracods as well as enhanced fluxes of small sized planktic foraminifera m notably g rubescens integrated over the year the species composition intercepted by the trap shows a very good match with the shell fauna in a core top sediment from the same site apparently the year estimated monsoonal record of planktic foraminifera which is strongly dominated by the sw monsoon production survives burial intact consequently the species composition of planktic foraminifera and measures such as the bulloides ruber ratio can be applied as a proxy for past changes in the intensity of the sw monsoon off somalia compared to the modern condition

primary production in the arabian sea during the last 135 000 years abstract variations in primary productivity pp have been reconstructed in eutrophic mesotrophic and oligotrophic parts of the arabian sea over the past 135 000 years applying principal component analysis and transfer function to planktic foraminiferal assemblages temporal variation in paleoproductivity is most pronounced in the mesotrophic northern nast site and oligotrophic eastern east site arabian sea and comparatively weak in the western eutrophic geob 3011 1 site in the upwelling area off oman higher pp during interglacials 250 320 g c m 2 year 1 than during cold stages 210 270 g c m 2 year 1 at geob 3011 1 could have been caused by a strengthened upwelling during intensified summer monsoons and increased wind velocities at nast during interglacials pp is estimated to exceed 250 g c m 2 year 1 and during glacials to be as low as 140 180 g c m 2 year 1 these fluctuations may result from a 1 varying impact of filaments that are associated to the oman coastal upwelling and 2 from open ocean upwelling associated to the findlater jet at east highest productivity of about 380 g c m 2 year 1 is documented for the transition from isotope stage 5 to 4 we suggest that during isotope stages 2 4 5 2 the transition 5 4 and the end of stage 6 deep mixing of surface waters was caused by moderate to strong winter monsoons and induced an injection of nutrients into the euphotic layer leading to enhanced primary production the deepening of the mixed layer during these intervals is confirmed by an increased concentration of deep dwelling planktic foraminiferal species a high productivity event in stage 3 displayed by estimated pp values and by planktic foraminifera and radiolaria flux and accumulation rate likely resulted from a combination of intensified sw monsoons with moderate to strong ne monsoons differential response of globigerina bulloides globigerinita glutinata and mixed layer species to the availability of food is suited to subdivide productivity regimes on a temporal and spatial scale

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

links between iron supply marine productivity sea surface temperature and co2 over the last 1 1 ma received 3 july 2008 revised 9 october 2008 accepted 27 october 2008 published 14 february 2009 1 paleoclimatic reconstructions have provided a unique data set to test the sensitivity of climate system to changes in atmospheric co2 concentrations however the mechanisms behind glacial interglacial g ig variations in atmospheric co2 concentrations observed in the antarctic ice cores are still not fully understood here we present a new multiproxy data set of sea surface temperatures sst dust and iron supply and marine export productivity from the marine sediment core ps2489 2 odp site 1090 located in the subantarctic atlantic that allow us to evaluate various hypotheses on the role of the southern ocean so in modulating atmospheric co2 concentrations back to 1 1 ma we show that antarctic atmospheric temperatures are closely linked to changes in so surface temperatures over the last 800 ka and use this to synchronize the timescales of our marine and the european project for ice coring in antarctica epica dome c edc records the close correlation observed between iron inputs and marine export production over the entire interval implies that the process of iron fertilization of marine biota has been a recurrent process operating in the subantarctic region over the g ig cycles of the last 1 1 ma however our data suggest that marine productivity can only explain a fraction of atmospheric co2 changes up to around 40 50 ppmv occurring at glacial maxima in each glacial stage in this sense the good correlation of our sst record to the edc temperature reconstruction suggests that the initial glacial co2 decrease as well as the change in the amplitude of the co2 cycles observed around 400 ka was most likely driven by physical processes possibly related to changes in antarctic sea ice extent surface water stratification and westerly winds position

paleoseismological evidence from lake sediments for recent movement on the denali and other faults yukon territory canada abstract most of the pacific north america plate motion in northwestern canada and southeastern alaska occurs along coastal faults that are the inland extension of the queen charlotte dextral transform fault more ancient faults farther east such as the denali and duke river faults exhibit moderate earthquake activity and may be accommodating part of the pacific plate motion paleoseismological results based on seismic resuspension of lake sediments covering a period of up to 3000 years show that the seismicity on the denali duke river fault system is remarkably recent spanning just the last three to five centuries there is a northerly trending zone of seismicity from the active fairweather contact fault system on the coast across to the part of the denali fault that is currently as well as prehistorically active this partial transfer eastward of pacific plate motion may be analogous to a similar proposed transfer of motion northward to the denali fault from the same fairweather contact fault system farther west in alaska

rock magnetic records of the sediments of the eastern arabian sea evidence for late quaternary climatic change abstract rock magnetic measurements along with grain size acid insoluble residue air organic carbon oc caco3 and 18o of the planktonic foraminifers of the sediments were determined for 15 gravity cores recovered from the western continental margin of india magnetic susceptibility ms values in the surficial sediments reflect the land derived input and in general are the highest in terrigenous sediment dominated sections of the cores off saurashtra ratnagiri followed by the sediments off indus gulf of kachchh and then mangalore cape comorin the down core variations in mineral magnetic parameters reveal that the glacial sediments off the indus are characterized by low ms values s ratios associated with high air content low oc caco3 contents and relatively high 18o values while those off sw india are characterized by low ms values high s ratio associated with low air content and relatively high oc caco3 and 18o values conversely the early holocene sediments of all cores are characterized by high ms values s ratio associated with high air content low oc caco3 contents and gradually decreased 18o values these results imply that during the last glacial maximum lgm the cores off northwestern india received abundant continental supply leading to the predominance of eolian fluvial sedimentation in the sw region the influence of hinterland flux is less evident during this period but convective mixing associated with the ne monsoon resulted in increased productivity during the early holocene intense sw monsoon conditions resulted in high precipitation on land which in turn contributed increased air content ms values in the continental margin sediments a shallow water core off kochi further suggests that the intense sw monsoon conditions prevailed until about 5 ka the late holocene organic rich sediments of the sw margin of india were however subjected to early diagenesis at different intervals in the cores therefore caution is needed when interpreting regional climatic change from down core changes in sediment magnetic properties

glacial interglacial co2 change the iron hypothesis several explanations for the 200 to 280 ppm glacial interglacial change in atmospheric co2 concentrations deal with variations in southern ocean phytoplankton productivity and the related use or nonuse of major plant nutrients an hypothesis is presented herein in which arguments are made that new productivity in today s southern ocean 7 4 1013g yr 1 is limited by iron deficiency and hence the phytoplankton are unable to take advantage of the excess surface nitrate phosphate that if used could result in total southern ocean new production of 2 3 1015 g c yr 1 as a consequence of fe limited new productivity holocene interglacial co2 levels preindustrial are as high as they were during the last interglacial 280 ppm in contrast atmospheric dust fe supplies were 50 times higher during the last glacial maximum lgm because of this fe enrichment phytoplankton growth may have been greatly enhanced larger amounts of upwelled nutrients may have been used and the resulting stimulation of new productivity may have contributed to the lgm drawdown of atmospheric co2 to levels of less than 200 ppm background information and arguments in support of this hypothesis are presented

surface and deep water variations in the northeast indian ocean during 34 6 ka bp evidence from carbon and oxygen isotopes of fossil foraminifera abstract 13c and 18o records from planktic globigerinoides ruber and benthic foraminifera cibicides spp were obtained from a deep sea sediment core sk157 18 collected in the central bay of bengal the chronology of this gravity core 11 59 n 90 01 e water depth 3069 m core length 230 cm has been established using six radiocarbon ages large variations in carbon and oxygen isotopes are suggestive of significant changes in surface and deep water of the bay of bengal during 34 6 ka bp planktic 18o values in core sk157 18 are significantly higher compared to cores in the western bay of bengal and the andaman sea a marked 18o decrease at 8 7 ka bp is suggestive of low salinity surface water in the central bay of bengal probably due to the intensification of the indian southwest monsoon ism an increasing 13c trend in g ruber is seen during 34 26 and 14 7 ka bp and decreasing 13c trend between 26 and 15 ka bp intervals however these 13c shifts in planktic foraminifera are not coherent with the glacial interglacial change and may have been caused by wind induced variability in surface water 13c the 18o and 13c values in benthic foraminifera show considerable variations during the holocene and glacial times last glacial maximum lgm to holocene 18o shift in benthic foraminifera 1 5 exceeds ice volume effect by 0 5 suggesting glacial deep water cooling of 2 c assuming no salinity change this glacial deep water cooling may have been caused by the introduction of cooler southern ocean deep water sodw large fluctuations in benthic 13c are due to the changes in source water characteristics during the holocene and last glaciation glacial benthic foraminifera exhibit pronounced decrease in 13c suggesting reduced contribution from the north atlantic deep water nadw and increased influx of sodw in addition oxidation of organic matter due to the reduced ventilation of deep water masses during the lgm may have contributed in the significant decrease in 13c

ice age terminations 230th dated oxygen isotope records of stalagmites from sanbao cave china characterize asian monsoon am precipitation through the ends of the third and fourthmost recent ice ages as a result am records for the past four glacial terminations can now be precisely correlated with those from ice cores and marine sediments establishing the timing and sequence of major events in all four cases observations are consistent with a classic northern hemisphere summer insolation intensity trigger for an initial retreat of northern ice sheets meltwater and icebergs entering the north atlantic alter oceanic and atmospheric circulation and associated fluxes of heat and carbon causing increases in atmospheric co2 and antarctic temperatures that drive the termination in the southern hemisphere increasing co2 and summer insolation drive recession of northern ice sheets with probable positive feedbacks between sea level and co2

interhemispheric atlantic seesaw response during the last deglaciation the asynchronous relationship between millennial scale temperature changes over greenland and antarctica during the last glacial period has led to the notion of a bipolar seesaw which acts to redistribute heat depending on the state of meridional overturning circulation within the atlantic ocean here we present new records from the south atlantic that show rapid changes during the last deglaciation that were instantaneous within dating uncertainty and of opposite sign to those observed in the north atlantic our results demonstrate a direct link between the abrupt changes associated with variations in the atlantic meridional overturning circulation and the more gradual adjustments characteristic of the southern ocean these results emphasize the importance of the southern ocean for the development and transmission of millennial scale climate variability and highlight its role in deglacial climate change and the associated rise in atmospheric carbon dioxide

planktonic foraminiferal and alkenone records of the last deglaciation from the eastern arabian sea the oxygen isotope record of planktonic foraminifera from tropical core md77194 eastern arabian sea exhibits a clear two step deglaciation with a brief 18o transient event in the tropics this 18o maximum could correspond to a cooling or to a change in the 18o content of sea water in this study past sea surface temperature sst and primary production pp are reconstructed from foraminiferal transfer functions and compared to values estimated from alkenone measurements sst and pp records from both proxies indicate a 1 5 2 5 c deglacial warming coupled with a pp decrease and a 0 5 1 c cooling during the younger dryas yd a detailed comparison between independent micropaleontological and geochemical proxies helps us identify potential biases and thus strengthen the paleo reconstructions

quantifying carbonate dissolution and calibration of foraminiferal dissolution indices in the somali basin two sediment traps moored off somalia in 1992 1993 collected similar settling fluxes of carbonate and siliceous shells formed by various plankton groups planktic foraminifera showed large seasonal variations with more than 74 of the total planktic foraminifera flux collected during the sw monsoon summer upwelling when globigerina bulloides was dominating along with globigerinita glutinata and neogloboquadrina dutertrei the intermonsoon and ne monsoon assemblages were dominated by globigerinoides ruber we used the trap records as our no dissolution reference for comparison with three boxcore recorders in order to quantify the carbonate dissolution along a depth transect dissolution increases downslope from station 905 to 907 and 915 at depths of 1567 m 2807 m and 4035 m respectively the carbonate fraction of the sediment at station 915 which is located near the ccd is the most affected by dissolution with more than 97 of the planktic foraminifera dissolved here the planktic foraminifera assemblage is strongly modified with thick walled species such as n dutertrei globorotalia tumida and pulleniatina obliquiloculata as the most resistant it is not representative of the settling assemblage the planktic foraminiferal assemblages of the sediment surface at stations 905 and 907 remain similar to the trap assemblages and the foraminifera are well preserved although only 25 of foraminifera are apparently preserved at station 905 and 8 at station 907 those numbers are surprisingly low and infer that only a small fraction of the foraminiferal carbonate production is buried and removed from the carbon cycle this discrepancy between the export and buried flux is partly be due to bio mechanical destruction by benthic processes and to supralysoclinal dissolution due to metabolic co2 generated by the benthic organisms another important factor is the interannual variability of the productivity well known in the arabian sea the calibration of commonly used foraminiferal dissolution indices percentage of foraminiferal fragments percentage of resistant species foraminiferal dissolution index fdx to our data only shows reliable results for high dissolution levels 97 off somalia the most accurate of the proxies is the percentage of foraminiferal fragments compared to the other methods tested i e fdx planktic foraminiferal loss l percentage of radiolarians and diatoms percentage of benthic foraminifera the species assemblage appears to be not significantly modified by dissolution unless the estimated shell loss is high 92 of the arriving shells in our samples this level is expressed in the percentage of fragmentation the percentage of radiolarians and diatoms the percentage of resistant foraminifera species and fdx as 80 35 25 and 1 8 respectively the relative abundance of globigerina bulloides is a valid sw monsoon upwelling proxy only when dissolution is moderate 92 globigerina bulloides and globigerinoides ruber have similar burial efficiencies and susceptibilities to dissolution thus the ratio g bulloides g ruber is a valid proxy for past changes in the intensity of the sw monsoon even in the strongly dissolved samples in our sediment record the ratio g bulloides g ruber indicates that the sw monsoon was stronger in the recent past than in 1992 93

can benthic foraminiferal morpho groups be used as indicators of paleomonsoonal precipitation a technique is proposed for the quick and easy assessment of paleomonsoonal precipitation through the study of morphological groups of foraminifera in a shallow water 20 m water depth sediment core collected off karwar near kali river mouth central west coast of india the percentage distribution of two morpho groups i e rounded symmetrical and angular asymmetrical morpho groups showed considerable fluctuations which correlate well with the 7 years average rainfall over a period of 116 years of the catchment area of kali river the dominance of rounded symmetrical forms indicates a period of higher rainfall whereas angular asymmetrical forms tend to increase in abundance during dry periods poor monsoons the inferences drawn shown high potential in generating proxy data for paleomonsoonal precipitation required to develop predictive climatic models

oxygen isotope evidence for a stronger winter monsoon current during the last glaciation a variety of palaeoclimate data from the northern indian ocean has shown that the asian summer monsoon was weaker during the last glacial maximum 18 000 years ago than it is today1 5 it has also been inferred from these data that the winter monsoon was stronger at that time5 here we present evidence that the transport of low salinity water from the bay of bengal to the arabian sea caused by the winter monsoon6 was higher during the last glacial maximum this inference is based on a negative excursion up to about 1 in magnitude in the oxygen isotope ratios of three planktonic foraminiferal species detected in a sediment core from the arabian sea by high resolution 1 2 cm sampling our results place an upper limit of about 4 000 years on the length of time during which the winter monsoon was stronger than today

influence of orographically enhanced sw monsoon flux on coastal processes along the se arabian sea 1 the arabian sea has an excess evaporation over precipitation regime the southeast arabian sea is however anomalous because it has 2800 4800 mm rainfall during the southwest monsoon swm despite a high rainfall the fluvial influence on supply of total suspended matter tsm and marine productivity is deemed insignificant and remains unevaluated we evaluated the poorly resolved influence of fluvial influx on shelf processes we documented low salinity stratification high productivity and turbidity over the entire inner shelf chlorophyll a 2 4 mg m 3 po4 3 2 5 m no3 6 8 2 1 m tsm 88 182 mg l 1 salinity near shore region 26 offshore region 33 8 34 6 p s u the deeper regions 40 m however had greatly reduced tsm and nutrient levels no3 po4 3 1 0 m tsm 24 mg l 1 and patches of high chlorophyll a upon cessation of the swm nutrient levels and tsm reduced considerable we identify two processes that contribute to the marine productivity and turbidity during the swm over the deeper regions there is a poor influence of fluvial supply and upwelling regulates productivity over the shallow inner shelf the sequestering of fluvial influx due to the prevalence of strong winds upwelling and equatorward flow is dominant the later processes induced high marine productivity and eutrophication in the near shore region and may have implications for siltation of channels reduced turbidity nutrient and chlorophyll a levels and higher salinity of the coastal waters during rest of the year imply a substantial role of fluvial fluxes on coastal processes

east west similarities and differences in the surface and deep northern arabian sea records during the past 21kyr abstract this study analyses monsoon proxy globigerina bulloides from ocean drilling program odp hole 723a western arabian sea and hole abp 25 02 northeastern arabian sea as well as dominant benthic foraminifera pteropods total organic carbon toc inorganic carbon ic and stable isotope values of benthic foraminifera from hole abp 25 02 to understand summer monsoon driven changes in the western and eastern arabian sea and their impacts on deep sea ventilation during the past 21 kyr we have also combined published toc data from odp hole 724b to understand if deep sea conditions in the arabian sea were same throughout the region during the studied interval the summer monsoon was generally weaker during cold intervals including the last glacial maximum the younger dryas and the bond events of the holocene from 20 to 10 kyr the deep northeastern arabian sea was better oxygenated with less organic carbon supply except during 14 12 5 kyr when toc values increased indicating a weaker summer monsoon and a weak oxygen minimum zone omz the toc values and the abundance of the eutrophic benthic foraminiferal species bulimina aculeata melonis barleeanum and uvigerina peregrina increased during the late holocene whereas well oxygenated low organic carbon benthic species sphaeroidina bulloides as well as pteropods decreased indicating an intense omz and an increased supply of refractory organic material to the arabian sea it is important to note that although the surface response to monsoon variability was more or less similar in the western and northeastern arabian sea the deep sea conditions show a marked contrast in the two regions during the past 21 kyr

coupled sea surface temperature seawater δ18o reconstructions in the arabian sea at the millennial scale for the last 35 ka two sediment cores from the western 905 10 46 9 n 51 56 4 e water depth 1586 m and eastern sk17 15 15 n 72 58 e water depth 840 m arabian sea were used to study past sea surface temperatures sst and seawater 18o 18ow variations for the past 35 ka we used coupled mg ca 18o calcite variability in two planktonic foraminiferal species globigerinoides ruber which thrives throughout the year and globigerina bulloides which occurs mainly when surface waters contain high nutrients during upwelling or convective mixing ssts in both areas based on mg ca in g ruber were 3 to 4 c lower during the last glacial maximum lgm 21 ka than today and the holocene period the sst records based on g bulloides also indicate general cooling down to 18 c in both areas ssts in the western arabian sea based on g bulloides were always lower than those based on g ruber indicating the presence of strong seasonal temperature contrast during the holocene and lgm we interpret the consistent presence of this seasonal temperature contrast to reflect a combination of seasonal summer upwelling sw monsoon and winter convective mixing ne monsoon in the western arabian sea in the eastern arabian sea g bulloides based ssts were slightly lower about 1 c than g ruber based ssts during the holocene indicating the almost absence of a seasonal temperature gradient similar to today however a large seasonal temperature contrast occurred during the lgm which favors the assumption that strong ne monsoon winds forced winter upwelling or convective mixing offshore goa sst and 18ow reconstructions reveal evidence of millennial scale cycles particularly in the eastern arabian sea here the stadial periods northern hemisphere cold periods such as younger dryas and heinrich events are marked by increasing ssts and salty sea surface conditions relative to those during the interstadial periods indeed the 18ow record shows evidence of low saline surface waters during interstadial periods indicating increased freshwater runoff from the western ghats as a consequence of enhanced sw monsoon intensity

a high resolution record of late quaternary upwelling along the oman margin arabian sea based on planktonic foraminifera planktonic foraminiferal abundances fluxes test sizes and coiling properties are influenced in various ways by the southwest monsoon winds and associated upwelling in the western arabian sea to determine the short term changes in the southwest monsoon we have carried out a high resolution time series analysis of three upwelling indices total flux of planktonic foraminiferal tests and flux and relative abundance of the planktonic foraminiferal species globigerina bulloides from ocean drilling program odp site 723a oman margin western arabian sea spanning the last 19 kyr in addition we have determined the relationships between upwelling intensity and the relative abundance fluxes and shell concentrations of various planktonic foraminiferal species upwelling indices suggest that from 19 to 16 ka 22 to 18 2 cal kyr b p the sw monsoon was relatively strong compared to the period 15 8 to 12 5 ka 17 8 to 13 8 cal kyr b p the intensification of the sw monsoon took place at 12 ka 13 1 cal kyr b p and reached a peak between 10 and 5 ka 10 6 and 4 8 cal kyr b p the high resolution data further demonstrate that the sw monsoon has started weakening from 5 ka 4 8 cal kyr b p and the weakest phase was in place at 3 5 ka 3 cal kyr b p which coincides with evidence of an arid climate in western tibet fluxes and shell concentrations of many of the planktonic foraminiferal species increased between 12 and 5 ka in response to the intensification of the sw monsoon winds after the last glacial period globigerina bulloides shows a fivefold to tenfold increase in flux during this period of intense upwelling the other species whose fluxes are influenced by this upwelling change are in order from strongest to weakest response globigerinita glutinata globigerinoides ruber neogloboquadrina dutertrei globigerinella aequilateralis globigerina falconensis and globigerinoides sacculifer the relative abundances of g bulloides and g ruber increased during intense upwelling whereas the relative abundances of g glutinata n dutertrei g falconensis and g sacculifer did not increase during this period which might be due to differences in the productivity of various species in relation to upwelling change therefore the fluxes and shell concentrations provide better and more reliable information about the changes in the monsoon system in the arabian sea than relative abundance data

wave induced stresses in unsaturated submarine sediments a simple method to solve the problem of pore pres sure and effective stresses induced by harmonic waves in a poroelastic sediment is presented by using biot s equation and a compatibility condition for elasticity the sediment is assumed to obey the effective stress principle in the normal definition for saturated sedi ments although it may contain gas bubbles occluded in the porewater the governing equation is a fourth order partial differential equation in terms of pore pressure or effective mean normal stress the solutions contain skempton s pore pressure coefficient b for unsaturated sediments with b 1 the pore pressure and effective stresses induced by wave action consist of two parts one part depends only on the wavelength and the other part depends on both the sediment and the wave characteristics the second part is out of phase with the wave when the submarine sediment is unsaturated the wave induced negative effective nor mal stresses become greater under the trough of waves a zone

late quaternary biogenic productivity and organic carbon deposition in the eastern arabian sea abstract paleoproductivity variations in the eastern arabian sea during the late quaternary period 42 ka bp to present have been studied using accumulation rates of sedimentary biogenic components organic carbon corg nitrogen n caco3 sr and ba such a multi proxy approach reveals decreased surface productivity during the last glacial interglacial transition the observed change of surface water productivity during the last glacial interglacial period is in antiphase to those observed in other low and mid latitude upwelling areas however consistent with some of the upwelling regions like nw africa and nw mexico sedimentary corg and n are found to be decoupled from surface productivity trend with significant enrichments in corg and n during the last glacial maximum lgm this is interpreted in terms of increase in sedimentation rates by a factor of 3 4 resulting in the better preservation of corg during the lgm

early deglacial 19 17 ka strengthening of the northeast monsoon 1 earlier studies that mostly relied on bulk 14c dating indicated a weaker southwest monsoon swm and a stronger than present northeast monsoon nem during the last glacial maximum lgm 18000 14c years or 21000 calendar years ago we have analyzed a sediment core from the equatorial indian ocean where planktonic foraminifera were dated by accelerator mass spectrometry ams 14c 18o analysis of three species of foraminifera viz globigerinoides ruber globigerinoides sacculifer and globorotalia menardii reveals that the nem strengthened during the early deglacial period 15000 14c years ago or 18000 calendar years ago relative to the present and not during the lgm as surmised earlier

carbonate and organic carbon content changes over last 20 ka in the southeastern arabian sea paleoceanographic implications two gravity cores aas 38 4 and aas 38 5 recovered from the eastern arabian sea were analyzed for calcium carbonate caco3 organic carbon aluminium al and titanium ti in order to understand the calcium carbonate and terrigenous fluctuations during the holocene and last glacial period high caco3 and low al and ti during the holocene and low caco3 and high al and ti during the last glacial period suggest that caco3 content in these two cores appears to be controlled by the dilution of terrigenous material the supply of terrigenous material to the core sites was higher during the last glacial period than in the holocene organic carbon values were lower 2 during the last glacial period in core aas 38 4 but the opposite was found in core aas 38 5 this inconsistent pattern of organic carbon changes in the two cores studied indicates that the distribution of organic carbon in the eastern arabian sea is controlled not only by the supply of organic matter from the water column but also by sediment texture and dilution of sediment components