Seasonal changes in coccolithophore densities in the Southern California Bight during 1991–1992

Seasonal changes in coccolithophore cell densities in the San Pedro Basin, Southern California Bight, were investigated for the period October 1991 to September 1992. Coccolitho phore cell densities ranged from 6.3 × 10⁴ coccospheres per liter in March 1992 to 0 cells per liter in November 1991. High coccolithophore concentrations occurred in late winter and spring, and low densities occurred in the summer and fall. The high coccolithophore densities during spring 1992 were associated with unusually low surface nutrient concentrations and a lack of upwelling, suggesting that the high densities were not part of a typical spring phytoplankton bloom in this region. We propose that the suppression of upwelling during spring 1992 may have been related to the prevailing ENSO conditions. Emiliania huxleyi type A dominated the total coccolithophore population throughout the year, Umbilicosphaera sibogae var. sibogae being the second most important contributor to the coccolithophore assemblages.     

A weighted surface-depth gradient method for the numerical integration of the 2D shallow water equations with topography

A finite volume MUSCL scheme for the numerical integration of 2D shallow water equations is presented. In the framework of the SLIC scheme, the proposed weighted surface-depth gradient method (WSDGM) computes intercell water depths through a weighted average of DGM and SGM reconstructions, in which the weight function depends on the local Froude number. This combination makes the scheme capable of performing a robust tracking of wet/dry fronts and, together with an unsplit centered discretization of the bed slope source term, of maintaining the static condition on non-flat topographies (C-property). A correction of the numerical fluxes in the computational cells with water depth smaller than a fixed tolerance enables a drastic reduction of the mass error in the presence of wetting and drying fronts. The effectiveness and robustness of the proposed scheme are assessed by comparing numerical results with analytical and reference solutions of a set of test cases. Moreover, to show the capability of the numerical model on field-scale applications, the results of a dam-break scenario are presented.     

Alkenones and coccoliths in ice‐rafted debris during the Last Glacial Maximum in the North Atlantic: implications for the use of UK37′ as a sea surface temperature proxy

The U^K₃₇′ index has proven to be a robust proxy to estimate past sea surface temperatures (SSTs) over a range of time scales, but like any other proxy, it has uncertainties. For instance, in reconstructions of the Last Glacial Maximum (LGM) in the northern North Atlantic, U^K₃₇′ indicates higher temperatures than those derived from foraminiferal proxies. Here we evaluate whether such warm glacial estimates are caused by the advection of reworked alkenones in ice‐rafted debris (IRD) to deep‐sea sediments. We have quantified both coccolith assemblages and alkenones in sediments from glaciogenic debris flows in the continental margins of the northern North Atlantic, and from a deep‐sea core from the Reykjanes Ridge. Certain debris flow deposits in the North Atlantic were generated by the presence of massive ice‐sheets in the past, and their associated ice streams. Such deposits are composed of the same materials that were present in the IRD at the time they were generated. We conclude that ice rafting from some locations was a transport pathway to the deep sea floor of reworked alkenones and pre‐Quaternary coccolith species during glacial stages, but that not all of the IRD contained alkenones, even when reworked coccoliths were present. We speculate that the ratio of reworked coccoliths to alkenone concentration might be useful to infer whether significant reworked alkenone inputs from IRD did occur at a particular site in the glacial North Atlantic. We also observe that alkenones in some of the debris flows contain a colder signal than estimated for LGM sediments in the northern North Atlantic. This is also clear in the deep‐sea core studied where the warmest intervals do not correspond to the intervals with large inputs of reworked coccoliths or IRD. We conclude that any possible bias to U^K₃₇′ estimates associated with reworked alkenones is not necessarily towards higher values, and that the high SST anomalies for the LGM are unlikely to be the result of a bias caused by IRD inputs. Copyright © 2011 John Wiley & Sons, Ltd.     

Late Glacial–Holocene ecostratigraphy of the south-eastern Aegean Sea, based on plankton and pollen assemblages

Quantitative analyses of coccolithophores, planktonic foraminifers, dinoflagellate cysts and pollen assemblages were carried out on shallow (NS-14) and deeper (NS-40) sediment cores from the south-eastern Aegean Sea. Nine coccolithophore (ACE 1–9) and nine planktonic foraminifer (APFE 1–9) ecozones, correlated with dinoflagellate cyst evidence, have been defined for the last ~14.5 cal. ka. Additionally, eight pollen assemblage zones (PAZ 1–8) have been recognised and correlated with the plankton ecozones. Although generally consistent with existing schemes for the central and eastern Mediterranean, the established high-resolution ecostratigraphy has led to an expanded palaeoecological reconstruction of the Late Glacial–Holocene archive in the south-eastern Aegean Sea, defining two warm and humid phases at 9.3–8.6 and 7.6–6.4 cal. ka b.p., associated with the deposition of the early Holocene sapropel S1, and a third one between 5.2 and 4.2 cal. ka b.p. The high sedimentation rates which characterise the study area enabled the detection of even minor and brief climatic events in the Aegean Sea during S1 deposition times. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.