As one of the most prominent seasonally recurring atmospheric circulation patterns, the Asian Sum... more As one of the most prominent seasonally recurring atmospheric circulation patterns, the Asian Summer Monsoon (ASM) plays a vital role for the life and livelihood of about a third of the global population. Changes in the strength and seasonality of the ASM significantly affect the region, yet the drivers of change and the varied regional responses of the ASM are not well understood. In the last two decades, there have been a number of studies reconstructing the ASM using stalagmite-based proxies such as oxygen isotopes (18O). Such reconstructions allow examination of the drivers and responses, increasing monsoon predictability. In this review paper, we focus on stalagmite 18O records from India at the proximal end of the ASM region. Indian stalagmite 18O records show well dated, high amplitude changes in response to the dominant drivers of the ASM on orbital to multi-centennial timescales and indicate the magnitude of monsoon variability in response to these drivers. We examine In...
As one of the most prominent seasonally recurring atmospheric circulation patterns, the Asian sum... more As one of the most prominent seasonally recurring atmospheric circulation patterns, the Asian summer monsoon (ASM) plays a vital role for the life and livelihood of about one-third of the global population. Changes in the strength and seasonality of the ASM significantly affect the ASM region, yet the drivers of change and the varied regional responses of the ASM are not well understood. In the last two decades, there were a number of studies reconstructing the ASM using stalagmite-based proxies such as oxygen isotopes (δ18O). Such reconstructions allow examination of ASM drivers and responses, increasing monsoon predictability. In this review paper, we focus on stalagmite δ18O records from India at the proximal end of the ASM region. Indian stalagmite δ18O records show well-dated, high-amplitude changes in response to the dominant drivers of the ASM on orbital to multi-centennial timescales, and indicate the magnitude of monsoon variability in response to these drivers. We examine ...
Stable carbon and oxygen isotopic investigations are carried out on planktonic and benthic forami... more Stable carbon and oxygen isotopic investigations are carried out on planktonic and benthic foraminifera from an AMS-dated sediment core of the northeast Indian Ocean (NEIO) to infer glacial to Holocene changes in surface and deep waters. The chronology of this gravity core (SK157-14; water-depth 3306m; lat. 5° 11′ N; long. 90° 05′ E) was established using six AMS radiocarbon ages and oxygen isotope stratigraphy. Variations in δ18O and δ13C values of planktonic (Globigerinoides ruber) and benthic foraminifera (Cibicidoides spp.) ...
Peritoneal metastases in patients with high-grade adenocarcinoma have been typically associated w... more Peritoneal metastases in patients with high-grade adenocarcinoma have been typically associated with a poor outcome. Recent literature has suggested that cytoreduction surgery (CRS) and hyperthermic intraperitoneal chemotherapy (HIPEC) may improve survival. We examined this subset of patients in an effort to better delineate those factors which contribute to improved survival. A retrospective review was performed looking at patients who had undergone CRS/HIPEC. Patients were identified as high-grade histology on the basis of pathology reports indicating their lesion as high grade, moderately, or poorly differentiated and/or associated with signet ring or goblet cell carcinoid features. Peritoneal cancer index and completeness of cytoreduction (CC) were used to define disease burden. Survival analysis was performed by the method of Kaplan-Meier with the log-rank test used to determine significance. Of the 250 patients who underwent CRS/HIPEC between 1999 and 2011, 36 (14%) were ident...
We present δ13C and δ18O and Mg/Ca records from planktonic (Globigerinoides ruber) and benthic fo... more We present δ13C and δ18O and Mg/Ca records from planktonic (Globigerinoides ruber) and benthic foraminifera (Cibicidoides spp.) from three sediment cores collected from the Bay of Bengal and the Arabian Sea. The chronology of these gravity cores is developed using AMS radiocarbon ages and oxygen isotope stratigraphy. Large variations in carbon and oxygen isotopes and Mg/ca ratios are suggestive of significant changes in surface and deep water masses since the last glaciation. Planktonic δ18O in the central and southern Bay of Bengal cores are significantly more enriched compared to sediment cores in the northern Bay of Bengal and the Andaman Sea because of the diminished influence of riverine fresh water fluxes. A marked δ18O decrease in the Bay of Bengal cores at 8-6 ka BP suggests intensified Indian monsoon precipitation that influenced the salinity structure in the entire Bay of Bengal. Spectral analysis of planktonic δ18O time series in the southern Bay of Bengal core (SK157-14)...
ABSTRACT We present δ13C and δ18O records of planktonic (Globigerinoides ruber) and benthic foram... more ABSTRACT We present δ13C and δ18O records of planktonic (Globigerinoides ruber) and benthic foraminifera (Cibicidoides spp.) from two deep sea sediment cores (SK157-15 and SK157-16) from the southern Bay of Bengal. The chronology of these gravity cores was established using eleven AMS radiocarbon ages from mixed species of planktonic foraminifera (Globigerinoides ruber and Globigerinoides sacculifer) and oxygen isotope stratigraphy. Large variations in δ13C and δ18O of planktonic and benthic foraminifera in these cores are suggestive of significant changes in surface and deep water masses for the ∼65 ka BP. The δ18O values in planktonic foraminifera in both these cores are significantly higher relative to sediment cores from the western Bay of Bengal and the Andaman Sea because of the reduced influence of riverine fresh water fluxes. Benthic δ13C values in cores SK157-15 and SK157-16 exhibit significant variations in source water characteristics during the Holocene and the last glaciation. Large decrease in δ13C values of bottom dwelling benthic foraminifera (Cibicidoides spp.) during the Last Glacial Maximum (LGM) and ∼50–65 ka BP suggest drastic reduction in North Atlantic Deep Water (NADW) fluxes into the southern Bay of Bengal and concurrent increase of deep water formed in the Southern Ocean (SODW) with significantly low δ13C values.
ABSTRACT δ13C and δ18O records from planktic (Globigerinoides ruber) and benthic foraminifera (Ci... more ABSTRACT δ13C and δ18O 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 δ18O values in core SK157–18 are significantly higher compared to cores in the western Bay of Bengal and the Andaman Sea. A marked δ18O 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 δ13C trend in G. ruber is seen during ∼34–26 and 14–7 ka BP and decreasing δ13C trend between 26 and 15 ka BP intervals. However these δ13C shifts in planktic foraminifera are not coherent with the glacial–interglacial change and may have been caused by wind-induced variability in surface water δ13C.The δ18O and δ13C values in benthic foraminifera show considerable variations during the Holocene and glacial times. Last Glacial Maximum (LGM) to Holocene δ18O 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 δ13C are due to the changes in source water characteristics during the Holocene and last glaciation. Glacial benthic foraminifera exhibit pronounced decrease in δ13C 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 δ13C.
ABSTRACT In this paper, sediment texture, clay mineral composition, and δ 18O data on Globigerino... more ABSTRACT In this paper, sediment texture, clay mineral composition, and δ 18O data on Globigerinoides ruber of a sediment core collected from a water depth of 250 m near Landfall Island, Bay of Bengal, is presented to understand paleoenvironmental shifts during the Mid–Late Holocene period. From the sediment core, five organic carbon-rich samples were radiocarbon dated and the reservoir-corrected ages range from 6,078 to 1,658 years BP. The marine sediment core is represented dominantly by clayey silt with incursions of coarser components that occur around 6,000, 5,400, and 3,400 years BP. The sedimentation of the coarser detritus is due to strengthened southwest monsoon (SWM) since 6,500 to 5,400 years BP. Clay minerals are represented by smectite, illite, kaolinite, and chlorite in varying amounts. High kaolinite content and K/C ratio indicate intense SWM and strong bedrock weathering from the hinterland (~6,500–5,400 years BP). Incidence of smectite (48.82 to 25.09 %) and chlorite/illite (C/I) ratio (0.56 to 0.28) indicate an overall weakened southwest monsoon since 6,000 to 2,000 years BP with a brief incursion of extremely reduced SWM around 4,400 to 4,200 years BP. This is corroborated with the oxygen isotope on G. ruber that reveals a significant shift in the isotopic values ~4,300 years BP (−3.39 ‰), indicating weakening in SWM. Subsequently, fluctuations in the intensity of SWM are observed since 2,000 years to present.
A high resolution record of the Indian summer monsoon (ISM) is generated using a δ18O time series... more A high resolution record of the Indian summer monsoon (ISM) is generated using a δ18O time series from a stalagmite collected from the Valmiki cave in southern India. This record covers a time span of ~ 1,000 years from 15,700 to 14,700 yr BP (before 1950 AD) with an average sampling resolution of ~ 5 years. High amplitude δ18O variation in this record reflects abrupt changes in ISM activity during the last deglaciation and suggest an age for the onset of Termination 1a (T1a) at ~ 14,800 yr BP in the Indian sub-continent. This record shows evidence for strong changes in tropical climate during the last deglaciation. Coincident variability in VSPM4 δ18O with speleothems from southern China during Termination 1a suggests that these caves reflect fluctuations in ISM activity. The variance in δ18O amplitude reveals significant multidecadal variability in ISM activity. Our record reveals intervals of strong monsoon activity during the later phase of Heinrich event 1 (H1) and shows synchr...
Sea surface temperature (SST) and seawater δ18Osw records were generated from sediment cores loca... more Sea surface temperature (SST) and seawater δ18Osw records were generated from sediment cores located in the southern Bay of Bengal (SBOB) and the northeastern Arabian Sea (NEAS) to understand glacial to Holocene changes in the hydrography of these regions. This was accomplished through the use of paired δ18O and Mg/Ca measurements in planktic foraminifera (Globigerinoides ruber) from two sediment cores; SK157-14 in the SBOB (lat. 5°11′N; long. 90°05′E; water-depth 3306 m) and SK148-21 in the NEAS (lat. 21°29′N, long. 67°01′E; water-depth 1900 m). The results suggest significant changes in SST and δ18Osw since last glacial period. The glacial SSTs in the SBOB and the NEAS were lower by ~ 2–2.5 °C relative to the Holocene. The deglacial period in both cores is characterized by significant variations in SSTs and seawater δ18Osw. The Dansgaard-Oeschger (DO) cycles and Heinrich events are strongly expressed in SST and δ18Osw records of the SBOB core. These records support a strong control of atmospheric-oceanic changes in the northern high latitude on thermal state of the SBOB. The SST time series in both these regions indicates a sudden and abrupt increase in temperature at the end of last glacial maximum. The SST time series in the NEAS core SK 148-21 reveals an intensification of the northeast monsoon during the last glacial maximum.
Pedogenic carbonates, widespread in paleosols throughout the geological ages, have been widely us... more Pedogenic carbonates, widespread in paleosols throughout the geological ages, have been widely used to infer climatic conditions and vegetation type during their formation. They are ubiquitous and widespread in the Late Quaternary loess–paleosol sequence of the Kashmir Valley, mostly in the form of carbonate nodules. In the present study, pedogenic carbonates are studied to analyze trends in the stable isotopic composition of carbonate nodules across three well-developed loess–paleosol sections in the Kashmir Valley. The stable carbon isotopic (δ13C) analysis reveals a gradual increase of the C4 vegetation towards the top of the sections. The development of C4 plant biomass towards the top of sections possibly reflects water stress and/or reduced atmospheric pCO2 and increasing aridity. The oxygen (δ18O) isotopic analysis reveals little variation with the values ranging from −6.37‰ to −7.75‰, reflecting stable climatic conditions during the development of pedogenic carbonate nodules. The use of geospatial data including digital elevation model (DEM) and Landsat TM remote sensing imagery have added morphological characterization of the topographic and landform features observed in the area. The tectonic uplift of the Pir Panjal Range, variation in the geometry of Karewa Basin and the slope gradient have played a key role in the spatial variation of loess–paleosol sequences in the valley. The Late Quaternary climate changes, deposition of loessic sediments, formation of the interbedded paleosol profiles and the subsequent denudation processes have resulted in the present day typical geomorphic landscape of the Kashmir Valley. The micromorphological analysis of loess–paleosol horizons reveals cold arid to semi-arid climatic conditions prevailed during the Late Quaternary in the valley.
A 17-year-long δ13C and δ18O record from a scleractinian coral (Porites spp.) of Bangaram island ... more A 17-year-long δ13C and δ18O record from a scleractinian coral (Porites spp.) of Bangaram island (Lakshadweep Archipelago) is based on a nearly-monthly sample interval. This live coral head of Porites spp. was recovered from the lagoon of Bangaram (Lakshadweep) island at ∼20 m water-depth. The skeletal δ13C and δ18O values varied approximately in phase and exhibit a positive relationship with each other. High-density (monsoon) bands are characterized by enriched δ18O values and low-density (non-monsoon) bands by depleted δ18O values. Coral δ13C and δ18O primarily reflects local oceanographic and climatic variability. The high-density bands were formed during the southwest monsoon (June–September) because of increased turbidity and cloud cover, whereas low-density bands were deposited during the non-monsoon months (October–May). Temporal changes in coral-derived sea surface temperature (SST) show a good agreement with instrumental SST record. The δ18O values in monsoon and non-monsoon bands are mainly due to the sea surface temperature (SST) changes, controlled by monsoon-induced upwelling and insolation changes. Coral-derived SST values clearly show warming events during summer 1993 and 1998. Spectral analysis of δ18O data reveals a teleconnection between the local SST and tropical Pacific climate variability. Enriched δ18O values for the monsoon months of 1984, 1985, 1993 and 1997 are probably indicative of cooler surface water due to the stronger upwelling at the studied location.Generally higher δ13C values correlate with enriched δ18O values of the monsoon bands. This increase in δ13C during the southwest monsoon months is attributed to a decrease in endosymbiotic photosynthesis. Alternatively, an increase in biological production, due to monsoon-induced upwelling, may have resulted in the δ13C enrichment of dissolved inorganic carbon (DIC) in surface waters. However, other possibilities, such as higher δ13C values due to coral mass-spawning events during monsoon times cannot be ruled out. A progressive decrease of ∼0.5‰ in δ13C from 1985 to 2001 may be due to the changes in photosynthesis and/or changing food habits from autotrophy to heterotrophy as the coral grew in size.
Palaeogeography Palaeoclimatology Palaeoecology, Jan 1, 2008
Stable carbon and oxygen isotopic records of planktonic (Globigerinoides ruber) and benthic foram... more Stable carbon and oxygen isotopic records of planktonic (Globigerinoides ruber) and benthic foraminifera (mostly Cibicidoides wuellerstorfi) from a deep-sea core in the northeast Indian Ocean are used to infer surface and deep water characteristics for the last ~ 60 kyr. The gravity core (SK-157-14) studied here was retrieved from the Ninetyeast Ridge (5°11′N; 90°05′E) at a water-depth of 3306 m. Chronology of the core was established using nine radiocarbon dates and oxygen isotope stratigraphy. Significant variations in δ18O during the last 2–60 kyr BP are suggestive of large changes in monsoonal precipitation over the Indian sub-continent. The last glacial maximum (LGM) to Holocene shift in planktonic foraminifera δ18O (1.64‰) is less than documented earlier from the Bay of Bengal cores. Two prominent negative δ18O excursions at ~ 8–7 and ~ 20–18 kyr BP are attributed to the sudden influx of freshwater as a result of intensified monsoonal precipitation. Large fluctuations in δ18O of G. ruber during the Holocene suggest variability in riverine input. Planktonic δ18O values show a combined effect of increased sea surface salinity and decreased sea surface temperature (SST) during the LGM. In contrast, the planktonic δ13C values are not linked to the glacial-to-Holocene transition.Comparison of the benthic δ18O and δ13C time series with those of a Pacific core (RC13-110) suggests a similar glacial deep water evolution. The LGM to Holocene δ18O shift in benthic foraminifera (mostly C. wuellerstorfi) exceeds the ice volume effect by ~ 0.5‰, indicating a glacial deep water cooling of ~ 2 °C, assuming no salinity change. Variations in the distribution of δ13C in the glacial northeast Indian Ocean (NEIO) are most likely the result of deep ocean circulation changes. The glacial deep NEIO δ13C characteristics clearly point to reduced North Atlantic Deep Water (NADW) input. Consequently the contribution from the Southern Ocean deep water may have increased resulting in low δ13C. A positive shift in δ13C during the early deglaciation is consistent with other records from this region. Deglacial δ13C fluctuations appear to have been caused by the switch ‘on’ and ‘off’ of NADW production.
As one of the most prominent seasonally recurring atmospheric circulation patterns, the Asian Sum... more As one of the most prominent seasonally recurring atmospheric circulation patterns, the Asian Summer Monsoon (ASM) plays a vital role for the life and livelihood of about a third of the global population. Changes in the strength and seasonality of the ASM significantly affect the region, yet the drivers of change and the varied regional responses of the ASM are not well understood. In the last two decades, there have been a number of studies reconstructing the ASM using stalagmite-based proxies such as oxygen isotopes (18O). Such reconstructions allow examination of the drivers and responses, increasing monsoon predictability. In this review paper, we focus on stalagmite 18O records from India at the proximal end of the ASM region. Indian stalagmite 18O records show well dated, high amplitude changes in response to the dominant drivers of the ASM on orbital to multi-centennial timescales and indicate the magnitude of monsoon variability in response to these drivers. We examine In...
As one of the most prominent seasonally recurring atmospheric circulation patterns, the Asian sum... more As one of the most prominent seasonally recurring atmospheric circulation patterns, the Asian summer monsoon (ASM) plays a vital role for the life and livelihood of about one-third of the global population. Changes in the strength and seasonality of the ASM significantly affect the ASM region, yet the drivers of change and the varied regional responses of the ASM are not well understood. In the last two decades, there were a number of studies reconstructing the ASM using stalagmite-based proxies such as oxygen isotopes (δ18O). Such reconstructions allow examination of ASM drivers and responses, increasing monsoon predictability. In this review paper, we focus on stalagmite δ18O records from India at the proximal end of the ASM region. Indian stalagmite δ18O records show well-dated, high-amplitude changes in response to the dominant drivers of the ASM on orbital to multi-centennial timescales, and indicate the magnitude of monsoon variability in response to these drivers. We examine ...
Stable carbon and oxygen isotopic investigations are carried out on planktonic and benthic forami... more Stable carbon and oxygen isotopic investigations are carried out on planktonic and benthic foraminifera from an AMS-dated sediment core of the northeast Indian Ocean (NEIO) to infer glacial to Holocene changes in surface and deep waters. The chronology of this gravity core (SK157-14; water-depth 3306m; lat. 5° 11′ N; long. 90° 05′ E) was established using six AMS radiocarbon ages and oxygen isotope stratigraphy. Variations in δ18O and δ13C values of planktonic (Globigerinoides ruber) and benthic foraminifera (Cibicidoides spp.) ...
Peritoneal metastases in patients with high-grade adenocarcinoma have been typically associated w... more Peritoneal metastases in patients with high-grade adenocarcinoma have been typically associated with a poor outcome. Recent literature has suggested that cytoreduction surgery (CRS) and hyperthermic intraperitoneal chemotherapy (HIPEC) may improve survival. We examined this subset of patients in an effort to better delineate those factors which contribute to improved survival. A retrospective review was performed looking at patients who had undergone CRS/HIPEC. Patients were identified as high-grade histology on the basis of pathology reports indicating their lesion as high grade, moderately, or poorly differentiated and/or associated with signet ring or goblet cell carcinoid features. Peritoneal cancer index and completeness of cytoreduction (CC) were used to define disease burden. Survival analysis was performed by the method of Kaplan-Meier with the log-rank test used to determine significance. Of the 250 patients who underwent CRS/HIPEC between 1999 and 2011, 36 (14%) were ident...
We present δ13C and δ18O and Mg/Ca records from planktonic (Globigerinoides ruber) and benthic fo... more We present δ13C and δ18O and Mg/Ca records from planktonic (Globigerinoides ruber) and benthic foraminifera (Cibicidoides spp.) from three sediment cores collected from the Bay of Bengal and the Arabian Sea. The chronology of these gravity cores is developed using AMS radiocarbon ages and oxygen isotope stratigraphy. Large variations in carbon and oxygen isotopes and Mg/ca ratios are suggestive of significant changes in surface and deep water masses since the last glaciation. Planktonic δ18O in the central and southern Bay of Bengal cores are significantly more enriched compared to sediment cores in the northern Bay of Bengal and the Andaman Sea because of the diminished influence of riverine fresh water fluxes. A marked δ18O decrease in the Bay of Bengal cores at 8-6 ka BP suggests intensified Indian monsoon precipitation that influenced the salinity structure in the entire Bay of Bengal. Spectral analysis of planktonic δ18O time series in the southern Bay of Bengal core (SK157-14)...
ABSTRACT We present δ13C and δ18O records of planktonic (Globigerinoides ruber) and benthic foram... more ABSTRACT We present δ13C and δ18O records of planktonic (Globigerinoides ruber) and benthic foraminifera (Cibicidoides spp.) from two deep sea sediment cores (SK157-15 and SK157-16) from the southern Bay of Bengal. The chronology of these gravity cores was established using eleven AMS radiocarbon ages from mixed species of planktonic foraminifera (Globigerinoides ruber and Globigerinoides sacculifer) and oxygen isotope stratigraphy. Large variations in δ13C and δ18O of planktonic and benthic foraminifera in these cores are suggestive of significant changes in surface and deep water masses for the ∼65 ka BP. The δ18O values in planktonic foraminifera in both these cores are significantly higher relative to sediment cores from the western Bay of Bengal and the Andaman Sea because of the reduced influence of riverine fresh water fluxes. Benthic δ13C values in cores SK157-15 and SK157-16 exhibit significant variations in source water characteristics during the Holocene and the last glaciation. Large decrease in δ13C values of bottom dwelling benthic foraminifera (Cibicidoides spp.) during the Last Glacial Maximum (LGM) and ∼50–65 ka BP suggest drastic reduction in North Atlantic Deep Water (NADW) fluxes into the southern Bay of Bengal and concurrent increase of deep water formed in the Southern Ocean (SODW) with significantly low δ13C values.
ABSTRACT δ13C and δ18O records from planktic (Globigerinoides ruber) and benthic foraminifera (Ci... more ABSTRACT δ13C and δ18O 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 δ18O values in core SK157–18 are significantly higher compared to cores in the western Bay of Bengal and the Andaman Sea. A marked δ18O 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 δ13C trend in G. ruber is seen during ∼34–26 and 14–7 ka BP and decreasing δ13C trend between 26 and 15 ka BP intervals. However these δ13C shifts in planktic foraminifera are not coherent with the glacial–interglacial change and may have been caused by wind-induced variability in surface water δ13C.The δ18O and δ13C values in benthic foraminifera show considerable variations during the Holocene and glacial times. Last Glacial Maximum (LGM) to Holocene δ18O 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 δ13C are due to the changes in source water characteristics during the Holocene and last glaciation. Glacial benthic foraminifera exhibit pronounced decrease in δ13C 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 δ13C.
ABSTRACT In this paper, sediment texture, clay mineral composition, and δ 18O data on Globigerino... more ABSTRACT In this paper, sediment texture, clay mineral composition, and δ 18O data on Globigerinoides ruber of a sediment core collected from a water depth of 250 m near Landfall Island, Bay of Bengal, is presented to understand paleoenvironmental shifts during the Mid–Late Holocene period. From the sediment core, five organic carbon-rich samples were radiocarbon dated and the reservoir-corrected ages range from 6,078 to 1,658 years BP. The marine sediment core is represented dominantly by clayey silt with incursions of coarser components that occur around 6,000, 5,400, and 3,400 years BP. The sedimentation of the coarser detritus is due to strengthened southwest monsoon (SWM) since 6,500 to 5,400 years BP. Clay minerals are represented by smectite, illite, kaolinite, and chlorite in varying amounts. High kaolinite content and K/C ratio indicate intense SWM and strong bedrock weathering from the hinterland (~6,500–5,400 years BP). Incidence of smectite (48.82 to 25.09 %) and chlorite/illite (C/I) ratio (0.56 to 0.28) indicate an overall weakened southwest monsoon since 6,000 to 2,000 years BP with a brief incursion of extremely reduced SWM around 4,400 to 4,200 years BP. This is corroborated with the oxygen isotope on G. ruber that reveals a significant shift in the isotopic values ~4,300 years BP (−3.39 ‰), indicating weakening in SWM. Subsequently, fluctuations in the intensity of SWM are observed since 2,000 years to present.
A high resolution record of the Indian summer monsoon (ISM) is generated using a δ18O time series... more A high resolution record of the Indian summer monsoon (ISM) is generated using a δ18O time series from a stalagmite collected from the Valmiki cave in southern India. This record covers a time span of ~ 1,000 years from 15,700 to 14,700 yr BP (before 1950 AD) with an average sampling resolution of ~ 5 years. High amplitude δ18O variation in this record reflects abrupt changes in ISM activity during the last deglaciation and suggest an age for the onset of Termination 1a (T1a) at ~ 14,800 yr BP in the Indian sub-continent. This record shows evidence for strong changes in tropical climate during the last deglaciation. Coincident variability in VSPM4 δ18O with speleothems from southern China during Termination 1a suggests that these caves reflect fluctuations in ISM activity. The variance in δ18O amplitude reveals significant multidecadal variability in ISM activity. Our record reveals intervals of strong monsoon activity during the later phase of Heinrich event 1 (H1) and shows synchr...
Sea surface temperature (SST) and seawater δ18Osw records were generated from sediment cores loca... more Sea surface temperature (SST) and seawater δ18Osw records were generated from sediment cores located in the southern Bay of Bengal (SBOB) and the northeastern Arabian Sea (NEAS) to understand glacial to Holocene changes in the hydrography of these regions. This was accomplished through the use of paired δ18O and Mg/Ca measurements in planktic foraminifera (Globigerinoides ruber) from two sediment cores; SK157-14 in the SBOB (lat. 5°11′N; long. 90°05′E; water-depth 3306 m) and SK148-21 in the NEAS (lat. 21°29′N, long. 67°01′E; water-depth 1900 m). The results suggest significant changes in SST and δ18Osw since last glacial period. The glacial SSTs in the SBOB and the NEAS were lower by ~ 2–2.5 °C relative to the Holocene. The deglacial period in both cores is characterized by significant variations in SSTs and seawater δ18Osw. The Dansgaard-Oeschger (DO) cycles and Heinrich events are strongly expressed in SST and δ18Osw records of the SBOB core. These records support a strong control of atmospheric-oceanic changes in the northern high latitude on thermal state of the SBOB. The SST time series in both these regions indicates a sudden and abrupt increase in temperature at the end of last glacial maximum. The SST time series in the NEAS core SK 148-21 reveals an intensification of the northeast monsoon during the last glacial maximum.
Pedogenic carbonates, widespread in paleosols throughout the geological ages, have been widely us... more Pedogenic carbonates, widespread in paleosols throughout the geological ages, have been widely used to infer climatic conditions and vegetation type during their formation. They are ubiquitous and widespread in the Late Quaternary loess–paleosol sequence of the Kashmir Valley, mostly in the form of carbonate nodules. In the present study, pedogenic carbonates are studied to analyze trends in the stable isotopic composition of carbonate nodules across three well-developed loess–paleosol sections in the Kashmir Valley. The stable carbon isotopic (δ13C) analysis reveals a gradual increase of the C4 vegetation towards the top of the sections. The development of C4 plant biomass towards the top of sections possibly reflects water stress and/or reduced atmospheric pCO2 and increasing aridity. The oxygen (δ18O) isotopic analysis reveals little variation with the values ranging from −6.37‰ to −7.75‰, reflecting stable climatic conditions during the development of pedogenic carbonate nodules. The use of geospatial data including digital elevation model (DEM) and Landsat TM remote sensing imagery have added morphological characterization of the topographic and landform features observed in the area. The tectonic uplift of the Pir Panjal Range, variation in the geometry of Karewa Basin and the slope gradient have played a key role in the spatial variation of loess–paleosol sequences in the valley. The Late Quaternary climate changes, deposition of loessic sediments, formation of the interbedded paleosol profiles and the subsequent denudation processes have resulted in the present day typical geomorphic landscape of the Kashmir Valley. The micromorphological analysis of loess–paleosol horizons reveals cold arid to semi-arid climatic conditions prevailed during the Late Quaternary in the valley.
A 17-year-long δ13C and δ18O record from a scleractinian coral (Porites spp.) of Bangaram island ... more A 17-year-long δ13C and δ18O record from a scleractinian coral (Porites spp.) of Bangaram island (Lakshadweep Archipelago) is based on a nearly-monthly sample interval. This live coral head of Porites spp. was recovered from the lagoon of Bangaram (Lakshadweep) island at ∼20 m water-depth. The skeletal δ13C and δ18O values varied approximately in phase and exhibit a positive relationship with each other. High-density (monsoon) bands are characterized by enriched δ18O values and low-density (non-monsoon) bands by depleted δ18O values. Coral δ13C and δ18O primarily reflects local oceanographic and climatic variability. The high-density bands were formed during the southwest monsoon (June–September) because of increased turbidity and cloud cover, whereas low-density bands were deposited during the non-monsoon months (October–May). Temporal changes in coral-derived sea surface temperature (SST) show a good agreement with instrumental SST record. The δ18O values in monsoon and non-monsoon bands are mainly due to the sea surface temperature (SST) changes, controlled by monsoon-induced upwelling and insolation changes. Coral-derived SST values clearly show warming events during summer 1993 and 1998. Spectral analysis of δ18O data reveals a teleconnection between the local SST and tropical Pacific climate variability. Enriched δ18O values for the monsoon months of 1984, 1985, 1993 and 1997 are probably indicative of cooler surface water due to the stronger upwelling at the studied location.Generally higher δ13C values correlate with enriched δ18O values of the monsoon bands. This increase in δ13C during the southwest monsoon months is attributed to a decrease in endosymbiotic photosynthesis. Alternatively, an increase in biological production, due to monsoon-induced upwelling, may have resulted in the δ13C enrichment of dissolved inorganic carbon (DIC) in surface waters. However, other possibilities, such as higher δ13C values due to coral mass-spawning events during monsoon times cannot be ruled out. A progressive decrease of ∼0.5‰ in δ13C from 1985 to 2001 may be due to the changes in photosynthesis and/or changing food habits from autotrophy to heterotrophy as the coral grew in size.
Palaeogeography Palaeoclimatology Palaeoecology, Jan 1, 2008
Stable carbon and oxygen isotopic records of planktonic (Globigerinoides ruber) and benthic foram... more Stable carbon and oxygen isotopic records of planktonic (Globigerinoides ruber) and benthic foraminifera (mostly Cibicidoides wuellerstorfi) from a deep-sea core in the northeast Indian Ocean are used to infer surface and deep water characteristics for the last ~ 60 kyr. The gravity core (SK-157-14) studied here was retrieved from the Ninetyeast Ridge (5°11′N; 90°05′E) at a water-depth of 3306 m. Chronology of the core was established using nine radiocarbon dates and oxygen isotope stratigraphy. Significant variations in δ18O during the last 2–60 kyr BP are suggestive of large changes in monsoonal precipitation over the Indian sub-continent. The last glacial maximum (LGM) to Holocene shift in planktonic foraminifera δ18O (1.64‰) is less than documented earlier from the Bay of Bengal cores. Two prominent negative δ18O excursions at ~ 8–7 and ~ 20–18 kyr BP are attributed to the sudden influx of freshwater as a result of intensified monsoonal precipitation. Large fluctuations in δ18O of G. ruber during the Holocene suggest variability in riverine input. Planktonic δ18O values show a combined effect of increased sea surface salinity and decreased sea surface temperature (SST) during the LGM. In contrast, the planktonic δ13C values are not linked to the glacial-to-Holocene transition.Comparison of the benthic δ18O and δ13C time series with those of a Pacific core (RC13-110) suggests a similar glacial deep water evolution. The LGM to Holocene δ18O shift in benthic foraminifera (mostly C. wuellerstorfi) exceeds the ice volume effect by ~ 0.5‰, indicating a glacial deep water cooling of ~ 2 °C, assuming no salinity change. Variations in the distribution of δ13C in the glacial northeast Indian Ocean (NEIO) are most likely the result of deep ocean circulation changes. The glacial deep NEIO δ13C characteristics clearly point to reduced North Atlantic Deep Water (NADW) input. Consequently the contribution from the Southern Ocean deep water may have increased resulting in low δ13C. A positive shift in δ13C during the early deglaciation is consistent with other records from this region. Deglacial δ13C fluctuations appear to have been caused by the switch ‘on’ and ‘off’ of NADW production.
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