Nutricline/Thermocline orbital variations in the Equatorial Pacific ODP Site 1240 as revealed by coccolithophores during the last 500 Ka

The ODP Site 1240 is located in the equatorial cold tongue in the Eastern Equatorial Pacific. The analysis of the calcareous nannofossil assemblages allow us to monitor fluctuations in the Nutricline/Thermocline over the last 500 Ka, using the relationship between the upper photic zone taxa (small placoliths) and the lower photic zone taxa (Florisphaera profunda), by means of the N ratio function. The variations in this N ratio pointed out two intervals with dominant higher relative abundances of small placoliths over Florisphaera profunda, and other interval with an abrupt increasing in the relative abundance of Florisphaera profunda over the small placoliths, controlled esentially by a precessional force, and during the last 177 Ka by a markedly excentricity componen

Variaciones de rango orbital en la dinámica de la Nutriclina/Termoclina en el Pacífico Ecuatorial durante los últimos 500Ka indicado por cocolitofóridos (Site 1240, Leg 202)

The ODP Site 1240 is located in the equatorial cold tongue in the Eastern Equatorial Pacific. The analysis of the calcareous nannofossil assemblages allow us to monitor fluctuations in the Nutricline/Thermocline over the last 500 Ka, using the relationship between the upper photic zone taxa (small placoliths) and the lower photic zone taxa (Florisphaera profunda), by means of the N ratio function. The variations in this N ratio pointed out two intervals with dominant higher relative abundances of small placoliths over Florisphaera profunda, and other interval with an abrupt increasing in the relative abundance of Florisphaera profunda over the small placoliths, controlled esentially by a precessional force, and during the last 177 Ka by a markedly excentricity component

Late Pleistocene paleoproductivity patterns during the last climatic cycle in the Guyana Basin as revealed by calcareous nannoplankton

15 pages, 4 figures.Variations in the assemblages and abundances of calcareous nannoplankton have allowed us to interpret changes in oceanic and atmospheric dynamics in the Guyana Basin, mainly linked to the southeast trades over the last climatic cycle. Records of the paleoproductivity index of coccolithophores (N ratio) allowed us to monitor the nutri-thermocline fluctuations. Additionally, nannofossil accumulation rates vary closely with the N ratio, indicating a strong correlation between these two paleoproductivity proxies. The dominance of upper (small Noelaerhabdaceae, Emiliania huxleyi and Gephyrocapsa oceanica), over lower photic zone dwellers (Florisphaera profunda) during Termination II and interglacial substages 5.1 and 5.3 is related to eutrophic conditions due to a shoaling of the nutri-thermocline as a consequence of enhanced southeast Trade Winds. This activated an upwelling at the continental margin of the Guyana Basin. Low N ratio values and the dominance of F. profunda over the glacial substages of MIS 5 and glacial MIS 2-4 are linked to a deep nutri-thermocline (deep stratification of the mixed layer), at times of low influence of the southeast Trade Winds, and a weak upwelling. However, the N ratio during MIS 2-4 was slightly higher than those seen for the MIS 4/5 boundary and glacial substages 5.2 and 5.4. These micropaleontological proxies follow the insolation at high northern latitude (65º N): the high N ratio and NAR data from the Guyana Basin during Termination II and inter glacials 5.1. and 5.3 are correlated with high insolation values, and low values of the N ratio and NAR during the MIS 4/5 boundary, glacials 5.2, 5.4 and MIS 2-4 are correlated with low insolation at the same latitudes. This situation suggests a link between the ITCZ, the southeast Trade Wind dynamics and the Northern Hemisphere climate changes during the last climatic cycle.The research discussed here was supported by Spanish projects CGL2005-00642/BTE, CGL2006-10593 from the Ministerio de Educación y Ciencia, A008C05 from the Consejería de Educación de la Junta de Castilla y León and by the Spanish Grant BES-2003-0010 from the Ministerio de Educación y Ciencia to G.-E. López-Otálvaro.Peer reviewe

Discorhabdus as a key coccolith genus for paleoenvironmental reconstructions (Middle Jurassic, Lusitanian Basin): Biometry and taxonomic status

International audienceBiometric analyses of Discorhabdus coccoliths were performed on a set of 29 samples taken from the marl/limestone couplets of the reference section of Cabo Mondego (Lusitanian Basin, west Portugal), Late Aalenian to Early Bajocian in age. Mixture analysis performed on the whole dataset revealed a bimodal length size distribution for the Discorhabus pool composed of D. striatus and D. ignotus. The size limit at 5 Î¼m proposed here can be reliably used to distinguish both populations, but optical characteristics, namely birefringence, form an additional criterion essential to discriminate the two species. An increase in Discorhabdus size occurred from the Late Aalenian to the Early Bajocian, due to a raise in size of the whole Discorhabdus pool and in the abundance of D. striatus. We propose that an increase in Discorhabdus size upward in the section is associated with a positive trend in paleoproductivity as supported by the increase of nannofossil accumulation rates as well as the δ13C values of bulk carbonates sampled in the Cabo Mondego section. However, the Discorhabdus size pattern may also represent a response to sea-surface temperature, salinity or an evolutionary trend. A relative supra-regional warming of sea-surface water masses is suggested by δ18O values of biogenic carbonates and apatites that were sampled in several Mediterranean Tethyan settings. Rise of sea-surface temperature may have triggered an increase in the abundances of D. striatus as well as of the size of the Discorhabdus pool. A hypothetical influence of changing sea-surface salinity on Discorhabdus size or species-specific Discorhabdus abundances is not supported by our micropaleontological data or by available geochemical data. Our results also reveal that the increase in Discorhabdus size matches the history of calcareous nannofossil turnover (radiation, diversity and increasing abundances) during the Middle Jurassic

Automated recognition of Mesozoic coccoliths: application to the quantification of the Aalenian/Bajocian pelagic carbonate production

International audienc

Automated recognition of Mesozoic coccoliths: application to the quantification of the Aalenian/Bajocian pelagic carbonate production

International audienc

Late Pleistocene palaeoproductivity patterns during the last climatic cycle in the Guyana Basin as revealed by calcareous nannoplankton

International audienc

Variations in Discorhabdus size during the Middle Jurassic in the Lusitanian Cabo Mondego section: a paleoenvironmental clue

International audienc

Late Pleistocene paleoproductivity patterns during the last climatic cycle in the Guyana Basin as revealed by calcareous nannoplankton

International audienceVariations in the assemblages and abundances of calcareous nannoplankton have allowed us to interpret changes in oceanic and atmospheric dynamics in the Guyana Basin, mainly linked to the southeast trades over the last climatic cycle. Records of the paleoproductivity index of coccolithophores (N ratio) allowed us to monitor the nutri-thermocline fluctuations. Additionally, nannofossil accumulation rates vary closely with the N ratio, indicating a strong correlation between these two paleoproductivity proxies. The dominance of upper (small Noelaerhabdaceae, Emiliania huxleyi and Gephyrocapsa oceanica), over lower photic zone dwellers (Florisphaera profunda) during Termination II and interglacial substages 5.1 and 5.3 is related to eutrophic conditions due to a shoaling of the nutri-thermocline as a consequence of enhanced southeast Trade Winds. This activated an upwelling at the continental margin of the Guyana Basin. Low N ratio values and the dominance of F. profunda over the glacial substages of MIS 5 and glacial MIS 2-4 are linked to a deep nutri-thermocline (deep stratification of the mixed layer), at times of low influence of the southeast Trade Winds, and a weak upwelling. However, the N ratio during MIS 2-4 was slightly higher than those seen for the MIS 4/5 boundary and glacial substages 5.2 and 5.4. These micropaleontological proxies follow the insolation at high northern latitude (65° N): the high N ratio and NAR data from the Guyana Basin during Termination II and interglacials 5.1. and 5.3 are correlated with high insolation values, and low values of the N ratio and NAR during the MIS 4/5 boundary, glacials 5.2, 5.4 and MIS 2-4 are correlated with low insolation at the same latitudes. This situation suggests a link between the ITCZ, the southeast Trade Wind dynamics and the Northern Hemisphere climate changes during the last climatic cycle