Pteropod Preservation Research Articles

The Heliconoides Modified Dissolution Index-HMDI: Do methane seepage environments affect the preservation state of Heliconoides inflatus?

Pteropods are holoplanktonic molluscs presently endangered because their fragile aragonitic shell is very sensitive to ocean acidification. The preservation of pteropods, and in particular of the mesopelagic Heliconoides (Limacina) inflatus, has been used to assess aragonite saturation state in Quaternary sediments (Limacina Dissolution Index, LDX) as an inferred proxy for climatic changes.Three deep-sea cores retrieved in 2016 during the EUROFLEETS2 SEMSEEP cruise offshore Israel are investigated to assess the preservation degree of H. inflatus (d'Orbigny, 1835) in representative environments across the base of the southeastern Mediterranean margin (cold water coral, active seepage pockmark and deep sea channel areas). In some core intervals, yellowish, recrystallized internal molds of this species are present. They have been previously observed in the Eastern Mediterranean, but the nature and origin of their colour was never explained. Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD) revealed that the recrystallized molds of the H. inflatus shells are formed by aragonite needles and high-Mg calcite crystals. The formation and preservation of these internal molds is induced by advective methane emissions, typically precipitating aragonite and High-Mg calcite phases close to the sediment–water interface. These molds were never used in previous applications of the LDX or in any existing ranking of pteropod preservation. Therefore, we have categorized these peculiar features combining previous pteropod ranking descriptions with the new observations to develop a modified biotic index (Heliconoides Modified Dissolution Index-HMDI).High values of the HMDI at the base of the pockmark core correspond to the interval where also active seepage-related features – e.g., bubble emissions, low-oxygen benthic foraminifera, chemosymbiotic molluscs, small fragments of authigenic carbonate crusts and slightly depleted δ13C values – occur, showing the applicability of the HMDI to methane seeps environments.

A ∼ 50 kyr record of carbonate (pteropods) preservation from the Laccadive Sea, Northern Indian Ocean

To evaluate the preservation pattern of pteropods and their relationship with climatic and oceanographic history in the Laccadive Sea, temporal variation analysis of pteropod abundance was carried out. We employed preservation indices from calcite (Globigerina bulloides%, Globorotalia menardii abundance), as well as aragonite (e.g., total pteropod abundance, Limacina Dissolution Index (LDX), fragmentation ratio) as a proxy. To determine if pteropod shells have been preserved over time, we used estimated pteropod abundance. The pteropod preservation record displays excellent preservation during cold stadials, evidenced by the lower values of aragonite dissolution proxies than during the interglacials/interstadials, similar to the preservation records from other northern Indian Ocean cores. The shallow aragonite compensation depth (ACD), weaker oxygen minimum zone (OMZ), and the lower southwest monsoon (SWM)-induced productivity is thought to be the cause of the basin-wide pteropod preservation events during the cold stadials (ACD). Additionally, during an intense northeast monsoon (NEM), the advection of cold, low-saline waters from the Bay of Bengal to the Laccadive Sea, as well as the intrusion of southern-sourced intermediate water ventilation, may have caused a deep vertical mixing of oxygen-rich surface waters, raised the pH of thermocline waters and deepened the ACD. However, the local fluctuations in the water mass properties, such as the increased productivity maxima, the intense OMZ, and shallow ACD, as well as changes in the aragonite, are responsible for the poor pteropod abundance, poor preservation and strong dissolution during the Holocene, Bølling-Allerød (B/A) and interstadial periods.The calcification proxy indicates that the aragonite undersaturation and reduced calcification occurred during 19–16.5 kyr, preferably due to the depletion in the oceanic alkalinity caused by enhanced upwelling-induced carbonate ion exchange between the intermediate and deep water. In contrast, the preferential dissolution of smaller shells in the sediments (marked by increased average shell size and higher values of Limacina dissolution index (LDX)) corresponds to strengthened OMZ and shallower ACD, pointing towards the post-depositional dissolution of aragonite shells. Therefore, the overall decrease of pteropod content of the deposits in the stadial/interstadials suggests a combination of monsoon-associated changes in water column properties, variability in aragonite saturation, intermediate water ventilation and sediment rate.

Centennial to millennial-scale changes in thermocline ventilation in the Arabian Sea: insights from the pteropod preservation record

The Arabian Sea hosts one of the three thickest oxygen minimum zones (OMZs) of the world ocean. Mid–depth oxygen depletion profoundly influences the chemistry of thermocline waters (HCO3ˉ, CO32ˉ and pH), which in turn significantly influences the preservation state of carbonates. The carbonate preservation is primarily controlled by the degree of saturation level of seawater with respect to the calcite and aragonite. The seawater in OMZ is undersaturated with respect to the aragonite (a metastable polymorph of CaCO3). Pteropod test being aragonitic in composition is therefore highly susceptible to the dissolution and dissolves completely below the aragonite compensation depth (ACD). Because of the current condition of intense OMZ due to high primary productivity, enhanced respiration of sinking organic carbon and reduced thermocline circulation; the ACD is shallow, lying in the middle of the OMZ. Hence, preservation record of pteropods in sea–floor sediment archives past changes in thermocline oxygen condition, carbonate chemistry, the ACD and OMZ intensity. High resolution records of various pteropod preservation indices (total pteropod abundance, transparent Limacina inflata abundance, fragmentation index) in a sediment core from the lower OMZ of the Indian margin (off Goa) enabled to investigate aragonite preservation/dissolution events and their links with the changes in ACD and OMZ intensity in the eastern Arabian Sea during the last 70 kyr BP. The proxy records reveal centennial to millennial scale changes in aragonite preservation condition in concert with Northern Hemisphere climatic events (Dansgaard–Oeschger (D–O) cycles and Heinrich events). The pteropod preservation spikes apparently correspond to the Northern Hemisphere cold events (D–O stadials and Heinrich events). Whereas, the pteropod tests were either poorly preserved or completely dissolved during the warm phases of D–O cycles (interstadials). The aragonite preservation events are attributed to the low monsoon induced productivity combined with the increased thermocline ventilation by Subantarctic Mode and Antarctic Intermediate Waters (SAMW–AAIW) resulting a weak OMZ and deeper ACD. The novel proxies (abundances of Globorotalia menardii, a planktic foraminifera and Styliola subula, a pteropod species) are used to gain better insights in to the variability of thermocline ventilation and OMZ intensity through time.

Palaeoceanographic significance of aragonitic pteropods in the Indian Ocean – a review

The Quaternary pteropod records published in the late 1990s from the Indian Ocean provided vital information on past oceanographic and climatic changes in this region. This paper reviews the published work on pteropod preservation and abundance in the Indian Ocean to evaluate the usefulness of this microfossil group in palaeoceanographic reconstructions. Their aragonitic shell chemistry makes them reliable indicators of ocean acidification, paleaoceanography and paleoclimate. An increased interest in studying this faunal group has arisen from recent findings on the vulnerability of aragonite shell preservation to human impacted water column chemistry changes. The compiled pteropod studies show that most of the pteropod databases contain information on abundance and distribution. However, the biogeochemical parameters and processes, such as aragonite production in the water column, their settling fluxes and the shell dissolution patterns are less explored. Despite the wide geographical range of pteropods and their significance in the marine food web, the ecology and taxonomy of the holoplanktonic gastropods in the Indian Ocean are poorly known. Though these pteropods are recovered in plankton tows (standing crop) and sediment traps (settling fluxes), the surface sediments in large parts of the northern Indian Ocean are completely devoid of them. Thus, assessing their fate in the water column and the sediment–water interface and their response to prevailing biogeochemical conditions are worth pursuing. Genomic studies and isotopic and elemental chemistry of living and fossil pteropods could help assess their response to ecological changes. To date, such modern techniques have been little utilized in studying Indian Ocean pteropods. A knowledge of their shell chemistry is crucial in the context of increased input of anthropogenic CO2 and the susceptibility of pteropod preservation to changing climate.

Aragonite pteropod abundance and preservation records from the Maldives, equatorial Indian Ocean: Inferences on past oceanic carbonate saturation and dissolution events

During the International Ocean Discovery Program (IODP) Expedition 359, a long continuous carbonate-rich sequence was recovered from the Inner Sea of Maldives. We investigated pteropod proxies (absolute abundance of pteropods species, total pteropods, epipelagic to mesopelagic ratio, fragmentation ratio, Limacina Dissolution Index (LDX), mean shell size variations of L. inflata) from Sites U1467 (water depth: 487 m) and U1468 (water depth: 521 m) to understand both surface and sub-surface paleoceanographic changes in the equatorial Indian Ocean and to improve our understanding of the factors responsible for pteropod preservation on longer timescales. A total of 15 species of pteropods were identified, and their downcore variations were documented from the core top to 707.49 mbsf in U1467 and from 447.4 to 846.92 mbsf in U1468. At the Site U1467, pteropod shells show high abundances/preservation up to a depth of 45 mbsf (~1.2 Ma), which is consistent with the presence of aragonite content in sediments (with the top 50 m bearing high aragonite content). Beyond 45 mbsf, only fragmented pteropod shells were seen down to 50 mbsf (corresponding to 1.5 Ma) followed by a total absence of pteropod shells and fragments from 50 mbsf (~1.5 Ma) to the end of the core at 846.92 mbsf (~24 Ma). A decrease in the SO42ˉconcentration and alkalinity in the interstitial fluid geochemistry is seen at these depths. The presence of dolomite content below 50 mbsf also indicates the alteration of aragonite into dolomite. Analyses of the carbonate preservation proxies reveal that the pteropods exhibit considerable fluctuation in abundance/preservation during the last 1.2 Myr. A good to moderate preservation (LDX: 2 to 3) is seen which correlates well with the fragmentation ratio but with an inverse relation with calcification rate. The proxies for in-life pteropod shell dissolution (average size of L. inflata and LDX) indicate that glacial periods (MIS 16, 14, 6, 4 and 2) have shown no signs of dissolution pointing better calcification under aragonite-saturated water column which is in good correlation with reduced atmospheric CO₂ concentration. Epipelagic/mesopelagic ratio indicates that the water column exhibited enhanced ventilation and mixing during glacial to interglacial periods, but intervals of intense stratification, a sign of poor ventilation or weakened circulation, was prevalent beyond MIS 14. The longest interval of poorest preservation was marked during MIS 11 and 13, which corresponds to the ‘Mid-Brunhes Dissolution Interval (MBDI).’ On a longer time scale, the abundances/preservation of pteropods in the Maldives seems to be controlled by changes in the seawater chemistry associated with monsoon productivity, water column ventilation, and atmospheric CO2 concentration.

A first look at factors affecting aragonite compensation depth in the eastern Arabian Sea

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 (≥63μm). Based on the pteropod preservation, we report that in the eastern Arabian Sea, ACD lies at a water depth of ≤525m, 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 %Corg. 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 %Corg. 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 %Corg. 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 green-house gas emissions.

Timing and preservation mechanism of deglacial pteropod spike from theAndamanSea, northeasternIndianOcean

The aragonite compensation depth (ACD) fluctuated considerably during the last glacial until theHolocene with a dominant pteropod preservation spike during the deglacial period, which is prominently seen in three well‐dated cores covering theAndamanSea, northeasternIndianOcean. The precise time period of the preservation spike of pteropods is not known but this knowledge is crucial for stratigraphical correlation and also for understanding the driving mechanism. Isotopic and foraminiferal proxies were used to decipher the possible mechanism for pteropods preservation in theAndamanSea. The poor preservation/absence of pteropods during theHolocene in theAndamanSea may have implications for ocean acidification, driven by enhanced atmospheric CO2concentration. Strengthening of the summer monsoon and the resultant high biological productivity may also have played a role in the poor preservation of pteropods. The deglacial pteropod spike is characterized by high abundance/preservation of the pteropods between ∼19 and 15 cal. kaBP,associated with very low atmospheric CO2concentration. Isotope data suggest the prevalence of a glacial environment with reduced sea surface temperature, upwelling and enhanced salinity during the pteropod preservation spike. Total planktic foraminifera andGlobigerina bulloidesabundances are low during this period, implying a weakened summer monsoon and reduced foraminiferal productivity. Based on the preservation record of pteropods, it is inferred that theACDwas probably deepest (>2900 m) at 16.5 cal. kaBP. The synchronous regional occurrence of the pteropod preservation spike in theAndamanSea and in the northwestern Indian Ocean could potentially be employed as a stratigraphic marker.

Abrupt climate-induced changes in carbonate burial in the Arabian Sea: Causes and consequences

We present high-resolution records of aragonite contents and pteropods abundance in two sediment cores (SK 17 and MD 76-131) within the Oxygen Minimum Zone (OMZ) of the eastern Arabian Sea. We show large increases in aragonite contents during glacial and particularly during stadials (Heinrich Events). Using aragonite content, pteropods abundance, organic carbon percentage and abundance of fertile species of planktonic foraminifer we demonstrate that aragonite contents in the eastern Arabian Sea primarily reflects preservation linked to the deepening of Aragonite Compensation Depth (ACD) in the Arabian Sea. We show that these aragonite preservation events correspond with time equivalents of Henrich Events when Arabian Sea experienced large declines in monsoon driven productivity and greater penetration of Antarctica Intermediate water (AAIW) into the Arabian Sea. Thus, pteropod preservation in the Arabian Sea appears to be the result of rapid climate change through atmospheric and oceanic teleconnections. We suggest that the role of aragonite carbonate production and burial in margins and the resultant CO2 climate feedback to rapid climate changes remains poorly constrained.

Late Quaternary record of pteropod preservation from the Andaman Sea

In order to understand the glacial to interglacial fluctuations in pteropod preservation and productivity during the late Quaternary (∼ 54 ka BP to present), we investigated pteropod, organic carbon (C org) and Globigerina bulloides abundance in a deep sea core from the western Andaman Sea. Pteropod abundance and preservation is similar to the “Indo-Pacific carbonate preservation type”, showing better preservation during glacial and poor preservation during interglacial periods. The core site appears to have remained below the aragonite compensation depth (ACD) throughout the Holocene, indicated by the total absence of pteropods. Maximum abundance and good preservation of pteropods was observed during stadials such as Younger Dryas (YD), Heinrich Events (HEs) and Last Glacial Maxima (LGM) indicating weaker oxygen minimum zone (OMZ) and deeper ACD. Furthermore, the high relative abundance of mesopelagic pteropods over epipelagic forms suggests a well ventilated water column with weak OMZ particularly during LGM, apparently driven by intense winter monsoon. Increased monsoon-driven productivity was observed during 45–40 ka, of early Marine Isotope Stage 2 (MIS 2, 24–22 ka), Bølling/Allerød (B/A; 15–13.5 ka), YD and late Holocene as evidenced by C org content and G. bulloides. Enhanced pteropod preservation of H1 associated with low C org content and G. bulloides suggests that reduced monsoonal driven productivity might have influenced pteropod preservation. Deglacial preservation spike in the Andaman Sea is consistent with other northern Indian Ocean records and elsewhere outside the Indian Ocean implying the event is global in nature, marked by deepening of ACD probably driven by enhancement of winter monsoon on local and changes in intermediate water circulation on regional scale.

Variation in production, input and preservation of metastable calcium carbonate off Somalia during the last 90,000 years

To improve our understanding of the Late Pleistocene and Holocene carbonate system of the western Arabian Sea a high-resolution sedimentary record off Somalia has been analysed. The 15.26-m-long piston core 905 comprises a complete record of the last 90,000 years. We have measured concentrations of carbonate minerals, i.e., aragonite, calcite, Mg-calcite, and element ratios (Sr/Ca) together with pteropod counts and an estimation of the preservation state of pteropod shells to trace temporal changes in carbonate production and preservation. The Sr/Ca ratio shows strong similarities to the aragonite percentage and the δ 18O record of the planktic foraminifer Neogloboquadrina dutertrei. High Sr/Ca ratios together with fragments of corals found in the coarse fraction indicate that most of the aragonite is of shallow water origin (high-Sr aragonite) and pteropods contribute much less than expected. High resuspensional input of shallow-marine aragonite occurs during sea-level highstands (interglacials) and low input during lowstands (glacials). The Mg-calcite concentration record resembles the whole pteropod abundance and pteropod shell preservation records confirming the use of Mg-calcite in combination with pteropod preservation proxies to reconstruct past fluctuations in carbonate dissolution. Preservation of aragonite and Mg-calcite increases during stadials, H-equivalents, YD and late Marine Isotope Stage (MIS) 3. During late MIS 5/early MIS 4 and in the Late Holocene absence of few pteropods as well as low Mg-calcite weight percentages point to strong dissolution of aragonite and Mg-calcite.

Recent and Late Quaternary pteropod preservation on the Pakistan shelf and continental slope

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.

Late Quaternary millennial-scale variability in pelagic aragonite preservation off Somalia

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 < 2, except for H5-equivalent with a poorer shell preservation, LDX > 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. On longer time scales, the pteropod abundance corresponds to the ‘Indo-Pacific carbonate preservation type’ with poor preservation during interglacials and better preservation during glacials. Late MIS 5 to early MIS 4 sections (84.1–64.8 ka) and the Late Holocene interval (6.5–0 ka) of core 905 contain only traces of pteropods. The early Holocene (9.2–6.5 ka) part is characterized by low pteropod amounts. Between 64.8 and 43.4 ka strong fluctuations occur and an intermediate average relative pteropod abundance is revealed. Between 43.4 and 9.2 ka the highest amounts in relative pteropod abundance in core 905 are observed. Besides the regional monsoonal influence on deepwater chemistry, changes in deepwater circulation occurring on glacial/interglacial and stadial/interstadial time scales might have affected pteropod preservation. However, it remains elusive whether 1) deep water formation in the Arabian Sea, 2) inflow of Glacial North Atlantic Intermediate Water or 3) change in water mass properties of the Circumpolar Deep Water (which is the water mass currently bathing this site) contributed to the observed pteropod preservation pattern.

Benthic foraminiferal response to variations in surface water productivity and oxygenation in the northern Arabian Sea

A benthic foraminiferal record of 120,000 year obtained from the base of the Oxygen Minimum Zone (OMZ) at a site in the northern Arabian Sea was studied. Quantitative benthic foraminiferal data were compared with palaeoproductivity indices (C org and Globigerina bulloides) and indices for bottom water oxygen concentration (Pteropod Preservation Index, Mo/Al, V/Al and Mn/Al). The benthic foraminiferal record revealed two distinct assemblages that show variations in the precession frequency band. A high diversity, low equitability assemblage proliferates in isotopic stages 1, 4 and 5, and has been interpreted to reflect relatively oxygenated bottom water conditions. This assemblage shows a strong covariance with minima in summer monsoon productivity. A low diversity, low equitability fauna, typically with a few species showing high dominances, is considered to reflect low bottom water oxygen conditions related to eutrophic surface water conditions. Dominating species in this assemblage are bolivinids, buliminids and globobuliminids. This assemblage dominates during precession-driven maxima in summer surface water productivity, and also during isotopic stages 2 and 3. We suggest that an intensified and colder glacial winter (NE) monsoon led to increased winter production, which superimposed on the precession-driven changes in surface water productivity, resulted in overall poorly oxygenated bottom water during glacial stages 2 and 3. A promising proxy for tracing winter productivity in the northern Arabian Sea is the planktonic foraminifer Globigerina falconensis. The benthic foraminiferal species Rotaliatinopsis seminvoluta in particular, is prolific under glacial conditions.

Carbonate preservation and climatic changes in the central Red Sea during the last 380 kyr as recorded by pteropods

Numerical abundances of pteropods and planktic foraminifera, and the mode of pteropod preservation, were determined in core KL 11 taken from the central part of the Red Sea. The abundance of pteropods (shells/g dry sediment) was compared to that of planktic foraminifera (% Pt/Pf + Pt) — a technique that permits detection of changes in carbonate preservation for the last 380 kyr. The numerical abundance of pteropods is influenced by the properties of the water column, and preservation is influenced by the bottom water. During the last ∼ 200 kyr (except during isotope stage 5.5) carbonates are in general well preserved. During this period the abundance pattern of the pteropods and planktic foraminifera is very similar and follows the climatic signal of the Red Sea with high numbers during the interglacial stages, changing to very low numbers during glacial maximum conditions. The similar abundance trends of the two groups, and between them and the δ 18O curve, indicates abundance is strongly linked to salinity. From isotope stage 6 to the bottom of the core the pteropods occur in low numbers, unlike the planktic foraminifera, which continue to display the high-amplitude glacial-interglacial cyclicity. The deviations, mainly during interglacial stages, between the abundance pattern of the two groups and the low % Pt/Pf + Pt values, indicate a significant change in carbonate preservation. Distinctive carbonate dissolution intervals are recognized in the Red Sea, correlating to large scale deep water dissolution events of the ‘mid-Brunhes dissolution cycle’ in the Indian Ocean. The anti-estuarine circulation pattern of the Red Sea prevents a direct connection between the deep water masses of the two oceans and rules out the likelihood that changes in the deep water circulation caused these carbonate dissolution events. The numerical variations between nonmigratory epipelagic and migratory mesopelagic pteropods were used to evaluate changes in the structure of the water column. Abundance maxima of mesopelagic pteropods, as in the recent Red Sea, indicate an aerated water column with ≥0.5 ml O 2/l oxygen concentrations at the minimum zone. Mesopelagic abundance maxima coincide commonly with negative monsoonal index values indicating a more aerated water column connected to increasing aridity in the Red Sea region. Abundance maxima of epipelagic pteropods indicate a strongly stratified water column, at times causing severe depletion in oxygen at intermediate water depths. Epipelagic peak events coincide often with positive monsoon index values implying an overall milder and more humid climate in the Red Sea, probably associated with enhanced precessional-controlled southwest monsoon activity.

“Pteropod preservation spike” and its significance in the Andaman Sea

“Pteropod preservation spike” and its significance in the Andaman Sea

Late quaternary pteropods in the South China Sea: carbonate preservation and paleoenvironmental variation

Pteropod fossil assemblages were studied in four sediment cores selected from the South China Sea (SCS) in order to investigate carbonate preservation change through late Quaternary glacial-interglacial cycles. These cores were selected at water depths (w.d.) below, near, and/or above the present Aragonite Compensation Depth (ACD), i.e. S050-37KL (2695 m w.d.), S049-8KL (1040 m w.d.), and, SCS-12 (543 m w.d.) and RC14-79 (706 m w.d.), respectively. The abundance of the aragonitic shells down core reveals an increase in pteropod preservation during glacial times in the cores which are below the present ACD (∼1000 m w.d.). Carbonate dissolution increasesin interglacials as in the Pacific open ocean, even at the water depths above the present lysocline where higher carbonate contents was found in interglacials. A carbonate preservation maximum, however, exists at the last deglaciation. At sites shallower than the present pteropod lysocline (∼800 m w.d.) carbonate dissolution shows little influence on the glacial-interglacial change in carbonate preservation. The glacial presence of pteropods on the northern continental slope reveals drastic ACD fluctuations through glacial-interglacial cycles, and sinking of the ACD to below or near 2700 m present water depths in the northern SCS during glacial periods. Pteropod faunal composition indicates a western Pacific open ocean origin, and variations in fossil assemblages correlate well with sea surface temperature change. As most of the straits around the SCS emerged due to lower glacial sea-level, subtropical-temperate water influence from the western Pacific open ocean increased during glacial periods through the remaining water passageway of the Bashi Strait in the northeastern SCS. Hence, decreases in both the tropical warm-water group and bathypelagic species, as well as increases of subtropical-temperate water species, provide further evidence that glacial-interglacial contrast in faunal composition is related to the lowered sea level which altered the configuration of the glacial SCS.

Episodic preservation of pteropod oozes in the deep Northeast Atlantic Ocean: Climatic change and hydrothermal activity

An up to 13 cm thick pteropod ooze covers the ocean floor of the Northeast Atlantic Ocean at Five stations between 44°N and 48°30′ N at around 25°W at water depths of between 2800 and 3200 m. Subsurface pteropod horizons occur at least at two different stratigraphic levels. Pteropod shells, both from the surface layers and from the subsurface horizons, are FeMn stained, with an intensity of staining that increases with age. AMS 14C ages of single pteropods are between 1700 yrs B.P. and the Present for the surface layer, and between 7600 and 3300 yrs B.P. for the upper horizon in the sediment. The Mn/Fe ratios of the pteropod coatings are between 0.24 and 0.40, indicating a hydrothermal source. Apparently, episodic pteropod preservations record pulses of hydrothermally derived, elevated Mn and Fe concentrations. We conclude that pteropod preservation is the combined result of episodic, climatically induced non-dissolution, and subsequent hydrothermally induced protective FeMn coating.

Late Quaternary Pteropod Preservation in Eastern North Atlantic Sediments in Relation to Changing Climate

AMS 14C measurements on pteropod shells from eastern North Atlantic deep-sea cores reveal distinct periods of aragonite preservation during the last 16,000 years. Most preservation spikes coincide with documented periods of climatic change on a scale of 2 × 101 to 2 × 103 years.

Quaternary paleo-oceanography, pteropod preservation and stable-isotope record of the Red Sea

An abundance of pteropod species, their mode of preservation, and stable isotope ratios of carbon and oxygen in benthic and planktic foraminifera have been determined in core MD76-140 from the northern Red Sea. Isotopic data and benthic foraminiferal species indicate changes in O 2 concentration in the bottom sediments. The glacial stages are characterized by an almost anoxic bottom, while the interglacial bottom was generally well ventilated. In general very well preserved pteropod shells occur in glacial oxygen-depleted intervals, and also on recent well ventilated bottoms. In the interglacial stages the preservation deteriorates, and in the most extreme cases there is a complete dissolution of the aragonitic shells. In these cases most of the assemblage is lost and the only traces of the original shells are their internal molds. These are cemented with high magnesium calcite (HMC) which is a more stable carbonate phase in bottom sediments of the interglacial. Dissolution of aragonite discriminates between large and small shells, the latter tend to be removed completely, while the larger ones may be preserved as internal HMC molds. Limacina inflata is more readily molded than L. trochiformis, and thus its relative abundance increases in poorly preserved assemblages. The state of aragonitic shell preservation depends on the degree of organic matter combustion in the sediments. Well preserved shells occur in sediments where the supply of organic matter is limited and O 2 levels are high. Preservation deteriorates with increasing supply of organic carbon, O 2 consumption and increasing production of CO 2. In this case the dissolved carbonate is reprecipitated as HMC cement. With further increase in organic matter supply and reduced ventilation, nearly all oxygen is consumed and NO 3 − and subsequently SO 4 2− become the oxidizing agents. This is followed by an increasing carbonate alkalinity and excellent aragonitic shell preservation.