Luc Beaufort

Sensitivity of coccolithophores to carbonate chemistry and ocean acidification

About one-third of the carbon dioxide (CO2) released into the atmosphere as a result of human activity has been absorbed by the oceans, where it partitions into the constituent ions of carbonic acid. This leads to ocean acidification, one of the major threats to marine ecosystems and particularly to calcifying organisms such as corals, foraminifera and coccolithophores. Coccolithophores are abundant phytoplankton that are responsible for a large part of modern oceanic carbonate production. Culture experiments investigating the physiological response of coccolithophore calcification to increased CO2 have yielded contradictory results between and even within species. Here we quantified the calcite mass of dominant coccolithophores in the present ocean and over the past forty thousand years, and found a marked pattern of decreasing calcification with increasing partial pressure of CO2 and concomitant decreasing concentrations of CO32−. Our analyses revealed that differentially calcified species and morphotypes are distributed in the ocean according to carbonate chemistry. A substantial impact on the marine carbon cycle might be expected upon extrapolation of this correlation to predicted ocean acidification in the future. However, our discovery of a heavily calcified Emiliania huxleyi morphotype in modern waters with low pH highlights the complexity of assemblage-level responses to environmental forcing factors.

Sea surface temperature and productivity records for the past 240 kyr in the Arabian Sea

Abstract Deep-sea sediments of two cores from the western (TY93-929/P) and the southeastern (MD900963) Arabian Sea were used to study the variations of the Indian monsoon during previous climatic cycles. Core TY93-929/P was located between the SW monsoon driven upwelling centres off Somalia and Oman, which are characterized by large seasonal sea surface temperature (SST) and particle flux changes. By contrast, core MD900963, was situated near the Maldives platform, an equatorial ocean site with a rather small SST seasonality (less than 2°C). For both cores we have reconstructed SST variations by means of the unsaturation ratio of C37 alkenones, which is compared with the δ18O records established on planktonic foraminifera. In general, the SST records follow the δ18O variations, with an SST maximum during oxygen isotope stage 5.5 (the Last Interglacial at about 120–130 kyr) and a broad SST minimum during isotope stage 4 and 3.3 (approximately 40–50 kyr). The SST difference between the Holocene and the Last Glacial Maximum (LGM) is of the order of 2°C. In both cores the SSTs during isotope stage 6 are distinctly higher by 1–2°C than the cold SST minima during the last glacial cycle (LGM and stage 3). To reconstruct qualitatively the past productivity variations for the two cores, we used the concentrations and fluxes of alkenones and organic carbon, together with a productivity index based on coccolith species (Florisphaera profunda relative abundance). Within each core, there is a general agreement between the different palaeoproductivity proxies. In the southeastern Arabian Sea (core MD900963), glacial stages correspond to relatively high productivity, whereas warm interstadials coincide with low productivity. All time series of productivity proxies are dominated by a cyclicity of about 21–23 kyr, which corresponds to the insolation precessional cycle. A hypothesis could be that the NE monsoon winds were stronger during the glacial stages, which induced deepening of the surface mixed layer and injection of nutrients to the euphotic zone. By contrast, the records are more complicated in the upwelling region of the western Arabian Sea (core TY93-929/P). This is partly due to large changes in the sedimentation rates, which were higher during specific periods (isotope stages 6, 5.4, 5.2, 3 and 2). Unlike core MD900963, no simple relationship emerges from the comparison between the δ18O stratigraphy and productivity records. The greater complexity observed for core TY93-929/P could be the result of the superimposition of different patterns of productivity fluctuations for the two monsoon seasons, the SW monsoon being enhanced during interglacial periods, whereas the NE monsoon was increased during glacial intervals. A similar line of reasoning also could help explain the SST records by the superimposition of variations of three components: global atmospheric temperature, and SW and NE monsoon dynamics.

Coarse fraction fluctuations in pelagic carbonate sediments from the tropical Indian Ocean: A 1500‐kyr record of carbonate dissolution

Appendix Table Al Is available with entire article onmicrofiche. Order from the American Geophysical Union, 2000Florida Avenue, N.W., Washington, D.C. 20009. DocumentP94-001;

The formation of Pliocene sapropels and carbonate cycles in the Mediterranean: diagenesis, dilution, and productivity

2.50. Payment must accompany order. We examined coarse fraction contents of pelagic carbonates deposited between 2000-and 3700-m water depth in the tropical Indian Ocean using Ocean Drilling Program (ODP) sites 722 (Owen Ridge, Arabian Sea) and 758 (Ninetyeast Ridge, eastern equatorial Indian Ocean), and four giant piston cores collected by the French R/V Marion Dufresne during the SEYMAMA expedition. Over the last 1500 kyr, coarse fraction records display high-amplitude oscillations with an irregular wavelength on the order of ∼500 kyr. These oscillations can be correlated throughout the entire equatorial Indian Ocean, from the Seychelles area eastward to the Ninetyeast Ridge, and into the Arabian Sea. Changes in grain size mainly result from changes in carbonate dissolution as evidenced by the positive relationship between coarse fraction content and a foraminiferal preservation index based on test fragmentation. The well-known “mid-Bruhes dissolution cycle”represents the last part of this irregular long-term dissolution oscillation. The origin of this long-term oscillation is still poorly understood. Our observations suggest that it is not a true cycle (it has an irregular wavelength) and we propose that it may result from long-term changes in Ca++flux to the ocean. Sites 722 and 758 δ18O records provide a high-resolution stratigraphy that allows a detailed intersite comparison of the two coarse fraction records over the last 1500 kyr. Site 722 (2030 m) lies above the present and late Pleistocene lysocline. The lysocline shoaled to the position of site 758 (2925 m) only during the interglacial intervals that occurred between about 300 and 500 ka (Peterson and Prell, 1985a). Despite these supralysoclinal positions of the two sites, short-term changes in coarse fraction contents are correctable from one site to another and probably result from regional (or global) dissolution pulses. By stacking the normalized coarse fraction records from sites 722 and 758, we constructed a Composite Coarse Fraction Index (CCFI) curve in which most of the local signals cancelled out. The last 800 kyr of this curve appear to compare extremely well with the Composite Dissolution Index curve from core V34-53 (Ninetyeast Ridge), which unambiguously records past variations of carbonate dissolution in the equatorial Indian Ocean (Peterson and Prell, 1985a). In the late Pleistocene the CCFI variations are mainly associated with glacial-interglacial changes. They show strong 100 and 41 kyr periodicities but no clear precession-related periodicities. As proposed earlier by Peterson and Prell (1985a), the lack of precession frequencies may suggest that the regional carbonate dissolution signal is driven by changes in deepwater circulation. We cannot totally reject the possibility, however, that low temporal resolution and/or bioturbation degrade somehow the precessional signal at ODP sites 722 and 758. In contrast, spectral density of dissolution cycles in the giant (53 m long) piston core MD900963 (Maldives area) displays clear maxima centered on the precession frequencies (23 and 19 kyr−1) as well as on the kyr−1 frequency but shows little power at the 100- kyr−1 frequency. These high-frequency changes most probably result from changes in surface productivity associated with monsoon variability. Dissolution at this site may be ultimately controlled by the oxidation of organic matter which appears to be incorporated into the sediments in greater quantity during periods of weak SW monsoon and/or increased dry NE monsoon.

Bundled turbidite deposition in the central Pandora Trough (Gulf of Papua) since Last Glacial Maximum : Linking sediment nature and accumulation to sea level fluctuations at millennial timescale

High-resolution micropaleontological (planktonic foraminifera and calcareous nannofossils) and geochemical (stable isotopes, organic carbon, Fe, P, S, Ca, Ba, Mn, and Al) records are presented for the first sapropel-containing carbonate cycle in the Pliocene of Sicily. The carbonate cycle is characterized by a gray to white to beige to white color layering typical of the marls of the Trubi formation. A faintly laminated sapropel is intercalated in the gray-colored bed of the carbonate cycle. CaCO3 content varies from 40% in the beige to 45-50% in the white layers. Lowest CaCO3 content of 25–30% is found in the gray layer and sapropel. Variations in carbonate and organic matter percentages can best be explained by changes in paleoproductivity rather than by variations in dilution and dissolution. Total productivity was highest during deposition of the gray layer and sapropel, as indicated by high organic carbon and Ba contents and high abundance of Globorotalia puncticulata. Carbonate production reached its highest values, however, during deposition of the white layers, as evidenced by enhanced abundances of planktonic foraminifera and nannofossils. The low carbonate content in the gray layer and sapropel is explained in terms of a collapse in carbonate production caused by extreme changes in the physical and biochemical properties of the water column, which in turn resulted in siliceous plankton and opportunistic foraminifers such as Globorotalia puncticulata outcompeting most calcareous organisms. The beige layer represents a low-productivity environment similar to the present-day eastern Mediterranean basin. Several mechanisms have previously been proposed to explain variations in productivity in the eastern Mediterranean. Both sapropels and gray layers were deposited at times when perihelion occurred in northern hemisphere summer. We envisage that the increase in seasonal contrast resulting from this orbital configuration enhanced winter mixing and stabilization of the water column during summer, both leading to favorable conditions for intensification of the spring bloom. In addition, a decrease in excess evaporation, as can be deduced from the δ18O record, led to shoaling of the pycnocline and reduced circulation, thus enhancing the availability of nutrients in the photic zone. Finally, enhanced precipitation and associated runoff should have caused an increase in river-borne nutrients.

The glacial ocean productivity hypothesis: the importance of regional temporal and spatial studies

Since Last Glacial Maximum (23-19 ka), Earth climate warming and deglaciation occurred in two major steps (Bolling-Allerod and Preboreal), interrupted by a short cooling interval referred to as the Younger Dryas (12.5-11.5 ka B. P.). In this study, three cores (MV-33, MV-66, and MD-40) collected in the central part of Pandora Trough (Gulf of Papua) have been analyzed, and they reveal a detailed sedimentary pattern at millennial timescale. Siliciclastic turbidites disappeared during the Bolling-Allerod and Preboreal intervals to systematically reoccur during the Younger Dryas interval. Subsequent to the final disappearance of the siliciclastic turbidites a calciturbidite occurred during meltwater pulse 1B. The Holocene interval was characterized by a lack of siliciclastic turbidites, relatively high carbonate content, and fine bank-derived aragonitic sediment. The observed millennial timescale sedimentary variability can be explained by sea level fluctuations. During the Last Glacial Maximum, siliciclastic turbidites were numerous when the lowstand coastal system was located along the modern shelf edge. Although they did not occur during the intervals of maximum flooding of the shelf (during meltwater pulses 1A and 1B), siliciclastic turbidites reappear briefly during the Younger Dryas, an interval when sea level rise slowed, stopped, or perhaps even fell. The timing of the calciturbidite coincides with the first reflooding of Eastern Fields Reef, an atoll that remained exposed for most of the glacial stages.

Millennial‐scale dynamics of the east Asian winter monsoon during the last 200,000 years

Abstract Higher ocean productivity has often been proposed to explain lower atmospheric carbon dioxide during the last glacial episodes. But recent consideration of marine cores from different areas show that higher local productivity can be postulated for interglacials as well as for glacial periods. Based on the detailed study of two piston cores from the northwest Africa upwelling system, the results presented here, including δ18O stratigraphy, organic carbon contents and fluxes, Ti Al ratios and grain size measurements, clearly indicate that the two cases of sedimentary records can even co-exist within a single upwelling system. This regional heterogeneity is presumably attributed to combined wind stress and sea-level changes that would induce different sedimentary records in the northern and in the southern part of the system. These results emphasize the importance to understand and to model the response of the main kinds of highly productive oceanographic regional systems which are spatially heterogeneous due to complex continent-ocean interactions, or to the presence of mobile hydrodynamic heterogeneities. For such an understanding it is not necessary to acquire a huge amount of core data throughout the world ocean, but to increase the density of cores as well as the regional-scale modelling efforts in systems such as coastal and equatorial upwelling areas, and the migration areas of the southern polar front.

Biomass burning and oceanic primary production estimates in the Sulu Sea area over the last 380 kyr and the East Asian monsoon dynamics

The primary productivity dynamics of the last 200,000 years in the Sulu Sea was reconstructed using the abundance of the coccolithophore Florisphaera profunda in the IMAGES MD97-2141 core. We find that primary productivity was enhanced during glacial periods, which we suggest is due to a stronger East Asian winter monsoon. During the last 80 kyr, eight significant increases in primary productivity (PP) in the Sulu Sea are similar to East Asian winter monsoon changes recorded in Chinese loess. The PP maxima are not linked with Heinrich events (HE) in the North Atlantic, although four PP peaks are synchronous with HE. The PP oscillations have frequencies near those of the Dansgaard-Oeschger cycles in Northern Hemisphere ice records and indicate a teleconnection of the East Asian winter monsoon with Greenland climate. In this Sulu Sea record the East Asian winter monsoon oscillates with periodicities of6, 4.2-3.4, 2.3, and 1.5 kyr. In particular, the 1.5 kyr cycle exhibits a strong and pervasive signal from stage 6 to the Holocene without any ice volume modulation. This stationarity suggests that the 1.5 kyr cycle is not driven by some high-latitude forcing.

Seasonal dynamics of calcareous nannoplankton on a West European continental margin: the Bay of Biscay

Abstract Coccolithophorid assemblages and micro-charcoal content were analysed in giant piston core MD97-2141 recovered in the Sulu Sea (Philippines). These proxies help to reconstruct respectively the dynamics of the oceanic primary production (PP) and biomass burning in that area. PP in the Sulu Sea intensifies during the East Asian winter monsoon (EAWM) and therefore PP constitutes a proxy for EAWM dynamics. Most of the precipitation in the Sulu Sea region occurs during the East Asian summer monsoon (EASM). Because the intensity of biomass burning is related to dryness of the surrounding area, the sedimentary micro-charcoal content can be used as an inverse proxy for EASM intensity. Our results show that the EAWM intensifies during glacial times in agreement with previous studies. Precessional forcing appears to act directly on EAWN because of the early response of PP in that frequency band. The micro-charcoal record exhibits complex dynamics, which we attribute to the competing influence of the long-term El Nino Southern Oscillation (ENSO)-like forcing and the glacial/interglacial cycle on EASM. These influences create an unusual frequency spectrum with power around 30 kyr and 19 kyr attributed to the non-linear response to the 100-kyr cycle (glacial) and the 23-kyr (ENSO) cycle. A factor of two increase in the amplitude of the micro-charcoal variability between 51 and 10 ka BP could correspond to Homo sapiens biomass burning in the style of the fire-stick farming of the Australian Aborigines. We also find, on precession cycles, an opposite phase between EASM and EAWM records and an advance of −δ 18 O and δ 18 O respectively by 2000 yr.

A multiproxy assessment of the western equatorial Pacific hydrography during the last 30 kyr

Abstract We analysed coccolithophorid and calcareous dinoflagellate assemblages from an 18-month (June 1990–August 1991) sediment trap record in the Bay of Biscay. With three trap deployments, the sampling resolution ranged from five to eight days. Characterisation of the assemblage dynamics is based on the use of statistical tools such as principal component analysis (PCA). The assemblages record seasonal and short-term events, implying that despite the dominance of lateral transport from the shelf, the traps faithfully record ecological dynamics. Summer species are Braarudosphaera bigelowii, Calcidiscus leptoporus (small), Coccolithus pelagicus, Emiliania huxleyi (closed), small Gephyrocapsa, Helicosphaera carteri, Pontosphaera japonica, Syracosphaera gr. molischii, Thoracosphaera heimii, Umbellosphaera tenuis. Autumn assemblages are characterised by the high frequency of C. leptoporus (large), Syracosphaera pulchra, Florisphaera profunda and E. huxleyi (open). Pontosphaera discopora and G. muellerae are most abundant during the winter whereas Umbilicosphaera sibogae peaks during spring. No other species shows a clear seasonal abundance pattern. The changes in assemblage composition are correlatable with changes in environmental parameters, such as wind, wave and light. Environmental dynamics, linked to seasonal succession, trigger a shift in relative abundance of morphotypes of E. huxleyi and C. leptoporus. The relations between species in the coccolithophorid community are characterised by a strong species dominance: (1) E. huxleyi dominated the assemblages (54–94%) and (2) the community followed a geometric distribution (in ranked dominance of species). The dominance increases during higher phytoplankton production as indicated by higher fluxes of diatoms, silicoflagellates and coccospheres of E. huxleyi.