P. Diz

DISTRIBUTION OF BENTHIC FORAMINIFERA IN COARSE SEDIMENTS, RÍA DE VIGO, NW IBERIAN MARGIN

The distribution of live benthic foraminifera (>63 μm) in box core samples from coarse sediments of the Ria de Vigo (NW Iberian margin) is mainly related to food availability and bottom current range. Live benthic foraminifera are dominated by small forms (63–125 μm). Faunal densities and diversity values suggest that very coarse substrates provide a favorable settlement for live benthic foraminifera. It is suggested that microalgae or bacterial biofilms that colonize shell surfaces are the main food resource for foraminifera. During upwelling events, the food supply to the bottom increases and the environment is colonized by opportunistic species. Moderate- to low-velocity bottom currents favor such pathways. The upwelling not only enhances the density of benthic foraminifera, but also allows the arrival of new species from the shelf. Attached forms are related to relatively high-velocity bottom currents and free living forms to moderate- to low-velocity bottom currents. Live benthic foraminifera inhabit a wide range of sediment depths and are not restricted to the uppermost centimeter. Under relatively high-velocity bottom current regimes, the vertical distribution of live benthic foraminifera is controlled by current strength. In coarse sediments, attached forms commonly considered strict epifaunal, and inhabiting above the disturbance depth, are considered “pseudoepifauna.” Below the disturbance depth, the sediment is not affected by currents and a “true” infauna is recognized.

Timing of the descent into the last Ice Age determined by the bipolar seesaw

We present planktonic foraminiferal fauna and isotope records from the SE Atlantic that highlight the nature of millennial-scale variability over the last 100 kyr. We derive an hypothesis-driven age model for our records based on the empirical link between variations in Greenland temperature, ocean circulation and carbonate preservation in the deep SE Atlantic. Our results extend earlier findings of an anti-phase (seesaw) relationship between north and south for the largest abrupt events of Marine Isotope Stage (MIS) 3-2 and the last deglaciation. In particular we find that Heinrich Stadials were paralleled by inferred southward shifts of the thermal Subtropical Front. These were followed by pronounced rebounds of the front with the return to interstadial conditions in the north. Our results also shed light on the mechanism of glaciation. In contrast to the last deglaciation, which was a globally symmetric change superposed by interhemispheric asynchronicity, we find that the descent into full glacial conditions at the onset of MIS 4 (~70 ka) displayed interhemispheric synchrony. We suggest that this globally synchronous descent into glacial MIS 4 was preconditioned by orbital changes but that the timing was ultimately determined by abrupt changes in ocean / atmosphere circulation patterns i.e. the bipolar seesaw.

Approaches and constraints to the reconstruction of palaeoproductivity from Cape Basin abyssal benthic foraminifera (South Atlantic)

The characteristics of benthic foraminiferal assemblages from well-oxygenated deep-sea settings have been used to reconstruct past productivity conditions. None of the different approaches that have been developed is without complications or applies in all settings. In this study we assess the use of benthic foraminifera (accumulation rates and assemblages composition) as proxies for palaeoproductivity changes during the last glacial period (25 – 95 ka) in an abyssal core located in the south of Cape Basin (41.1 °S, 7.8 °E, 4981 m water depth). Assemblage characteristics indicate a generally food-limited environment receiving episodic inputs of labile organic carbon of variable strength. High seasonality in the delivery of organic material to the seafloor in the form of phytodetritus influences the assemblage characteristics because the corresponding response does not involve the whole community. Benefiting from this occasionally high organic input is the opportunistic species Epistominella exigua (Brady) that reproduces rapidly to build up large populations. In general, the rest of the species (i.e. less opportunistic compared to E. exigua) show only subtle variations in their population densities and fauna composition. Under those circumstances benthic foraminiferal accumulation rates seem to be independent of the amount of organic flux arriving at the sediment surface and respond instead to the strength of phytoplankton blooms.

Linkages between rapid climate variability and deep-sea benthic foraminifera in the deep Subantarctic South Atlantic during the last 95 kyr

We present a high-resolution record of benthic foraminifera fauna from a sediment core retrieved from the South Cape Basin (Subantarctic South Atlantic) spanning the last glacial cycle (95 kyr). Information provided by benthic foraminiferal assemblages together with paleoclimate proxies from the same core allow us to interpret changes in the style of primary production (episodic versus sustained) in relation to abrupt climate oscillations. Our results indicate that fluctuations in the abundance of the phytodetritus-related species, Epistominella exigua, are concomitant with millennial-scale high-latitude climate perturbations. Episodic phytoplankton blooms increased during a negative mode of the bipolar seesaw, irrespective of the magnitude of the perturbation (i.e., Heinrich stadial versus non-Heinrich stadial events). We provide a hypothesis linking the frequency and intensity of these events to atmospheric perturbations, interhemispheric climate variability, and millennial-scale changes in atmospheric CO2. A notable exception to the overall pattern is the generally high abundance of E. exigua across the globally synchronous onset of glacial marine oxygen isotope stage (MIS) 4, a period generally characterized by increased dustiness and low-quality organic carbon as inferred by the percentage of the nonphytodetritus species. This highlights the special characteristics governing the onset of MIS 4 in the Subantarctic.

Textural Characteristics might Influence Donax trunculus Shellfishing Banks Exploitability

In this study we combine textural and compositional information to characterize the intertidal sediments from three relevant shellfishing banks located in the Rías Altas (Vilarrube, Lombo das Navallas) and Rías Baixas (Playa America) of Galicia (NW Spain). The purpose of this study is to explore the relevance of the sediment characteristics as a key parameter in the decline of commercially exploitable populations of the bivalve Donax trunculus in two of the banks (Lombo das Navallas and Playa América) but not in the other (Vilarrube). The sediments of the three areas show their own distinctive sedimentary characteristics that became defined by their grain size parameters, mineralogical and elemental composition. This is the result of regional land supplies from the erosion of drainage areas and sea cliffs together with the contribution of modern carbonate bioclasts. Notably, the sediments of Vilarrube are characterized by moderately sorted to well sorted fine sand showing little spatial variability. According to our results, sediment texture seems to be a relevant parameter for the settlement and growth of commercial populations of Donax trunculus.