Vanishing coccolith vital effects with alleviated carbon limitation

By recreating a range of geologically relevant concentrations of dissolved inorganic carbon (DIC) in the laboratory, we demonstrate that the magnitude of the vital effects in both carbon and oxygen isotopes of coccolith calcite of multiple species relates to ambient DIC concentration. Under high DIC levels, all the examined coccoliths exhibit significantly reduced isotopic offsets from inorganic calcite compared to the substantial vital effects expressed at low (preindustrial and present-day) DIC concentrations. The supply of carbon to the cell exerts a primary control on biological fractionation in coccolith calcite via the modulation of coccolithophore growth rate, cell size and carbon utilisation by photosynthesis and calcification, altogether accounting for the observed interspecific differences between coccolith species. These laboratory observations support the recent hypothesis from field observations that the appearance of interspecific vital effect in coccolithophores coincides with the long-term Neogene decline of atmospheric CO2 concentrations and bring further valuable constraints by demonstrating a convergence of all examined species towards inorganic values at high pCO2 regimes. This study provides palaeoceanographers with a biogeochemical framework that can be utilised to further develop the use of calcareous nannofossils in palaeoceanography to derive sea surface temperature and pCO2 levels, especially during periods of relatively elevated pCO2 concentrations, as they prevailed during most of the Meso-Cenozoic.

Chemobiostratigraphy of the Cretaceous/Paleocene boundary at ODP Hole 120-750A

An integrated biostratigraphic and stable isotope investigation was conducted on a high-latitude sequence across the Cretaceous/Paleogene (K/P) boundary recovered in Hole 750A in the southern Indian Ocean. The sequence consists of nannofossil chalk and is discontinuous across the boundary; missing is an estimated 0.3-m.y. late Maestrichtian and early Danian interval. Nonetheless, because calcareous nannofossil Zones NP1 and NP2 are well-developed, micropaleontological studies of the sequence have yielded a detailed record of Danian high-latitude microplankton evolution. In addition, stable carbon isotope analyses of planktonic and benthic foraminifer and bulk samples provide a record of late Maestrichtian and early Danian surface- and deep-water carbon isotope variations. Together, the carbon isotope and carbonate accumulation records serve as an index of regional marine net productivity across the boundary. Earliest Danian nannoplankton assemblages consisted mainly of persistent genera that were generally rare or absent in the Upper Cretaceous at Hole 750A. However, by 0.5-0.6 m.y. after the boundary, newly evolving Danian taxa became dominant. The turnover in nannofossil assemblages was accompanied by significant changes in rates of net productivity as gauged by carbon isotope distributions and carbonate accumulation rates. During the period dominated by persistent taxa, net productivity was extremely low, as reflected by the absence of vertical delta13C gradients and reduced carbonate accumulation rates. Later in the Danian, as new species evolved and flourished, vertical delta13C gradients reappeared and carbonate accumulation rates increased, signaling partial recovery of net productivity in this region. The absolute timing and magnitude of late Maestrichtian and early Danian biotic and geochemical changes in the southern Indian Ocean were similar to those recorded in other pelagic K/P boundary sequences from low- and mid-latitude Atlantic and Pacific sites, indicating that these events were ubiquitous.

Raw data of SCADCP (self-contained Acoustic Doppler Current Profiler) from three moorings in the Eastern Weddell Sea, 2005-2008

The success of any efforts to determine the effects of climate change on marine ecosystems depends on understanding in the first instance the natural variations, which contemporarily occur on the interannual and shorter time scales. Here we present results on the environmental controls of zooplankton distribution patterns and behaviour in the eastern Weddell Sea, Southern Ocean. Zooplankton abundance and vertical migration are derived from the mean volume backscattering strength (MVBS) and the vertical velocity measured by moored acoustic Doppler current profilers (ADCPs), which were deployed simultaneously at 64°S, 66.5°S and 69°S along the Greenwich Meridian from February, 2005, until March, 2008. While these time series span a period of full three years they resolve hourly changes. A highly persistent behavioural pattern found at all three mooring locations is the synchronous diel vertical migration (DVM) of two distinct groups of zooplankton that migrate between a deep residence depth during daytime and a shallow depth during nighttime. The DVM was closely coupled to the astronomical daylight cycles. However, while the DVM was symmetric around local noon, the annual modulation of the DVM was clearly asymmetric around winter solstice or summer solstice, respectively, at all three mooring sites. DVM at our observation sites persisted throughout winter, even at the highest latitude exposed to the polar night. Since the magnitude as well as the relative rate of change of illumination is minimal at this time, we propose that the ultimate causes of DVM separated from the light-mediated proximal cue that coordinates it. In all three years, a marked change in the migration behaviour occurred in late spring (late October/early November), when DVM ceased. The complete suspension of DVM after early November is possibly caused by the combination of two factors: (1) increased availability of food in the surface mixed layer provided by the phytoplankton spring bloom, and (2) vanishing diurnal enhancement of the threat from visually oriented predators when the illumination is quasi-continuous during the polar and subpolar summer. Zooplankton abundance in the water column, estimated as the mean MVBS in the depth range 50-300 m, was highest end of summer and lowest mid to end winter on the average annual cycle. However, zooplankton abundance varied several-fold between years and between locations. Based on satellite and in situ data of chlorophyll and sea ice as well as on hydrographic measurements, the interannual and spatial variations of zooplankton mean abundance can be explained by differences in the magnitude of the phytoplankton spring bloom, which develops during the seasonal sea ice retreat. Whereas the vernal ice melt appears necessary to stimulate the blooming of phytoplankton, it is not the determinator of the blooms magnitude, its areal extent and duration. A possible explanation for the limitation of the phytoplankton bloom in some years is top-down control. We hypothesise that the phytoplankton spring development can be curbed by grazing when the zooplankton had attained high abundance by growth during the preceding summer.