Palaeoclimatic and biotic changes during the Aalenian (Middle Jurassic) at the southern Laurasian Seaway (Basque–Cantabrian Basin, northern Spain)

Abstract

The uppermost Toarcian-lowermost 8ajocian deposits have been studied in 10 sections located in the western part of the Basque-Cantabrian Basin (northern Spain). The studied 276 successive recorded ammonite assemblages allowed detailed biostratigraphical subdivision and the correlation of the sections at the ammonite zone and subzone scale. The foraminifers were studied in 96 samples and 55 diagenetically screened belemnite rostra were analyzed for C and 0 isotope.Facies and thickness distribution of the Aalenian sediments suggest that deposition took place in a nearly symmetrical sub-basin included into an intra plate shallow platform, developed over continental crust, on which flexure was the main control responsible for subsidence and basin evolution.Noteworthy negative 613Cb�1 excursions, coinciding with seawater temperature changes, were recorded during the Bradfordensis Biochron, around the Concavum-Limitatum biochron boundary, and around the Aalenian-Bajocian boundary. A close relationship between the changes in seawater temperature and the biotic changes observed in the foraminiferal and in the ammonoids assemblages has been evidenced. During the Comptum Biochron, the 6180�1-based palaeotemperature shows a remarkable cooling interval with an average seawater temperature of 15.7 QC. This cooling favoured the immigration of species of foraminifers that thrived in platforms with colder seawater temperatures located north of the Basque-Cantabrian Basin.As a consequence, a strong increase in the diversity of the foraminiferal assemblages (28.5% of first appearances) occurred. This interval also coincides with the highest ammonoid abundance recorded during the Aalenian. A notable increase in temperature with peak values up to 24.3 QC, was measured during the Bradfordensis Biochron. This warming marks the beginning of progressive loss of foraminifer diversity and the decrease in the abundance of ammonoids. However, the increase in the relative abundances of some foraminiferal taxa such as the genus Spirillina during the warming phase is remarkable. Some of the foraminiferal species that had their last occurrence during this interval seem to be immigrants from NW Europe that arrived during the Comptum cooling interval, and that did not survive to rise of the seawater temperature.Another drop in temperature was recorded during the late Gigantea and the early Concavum biochrons.Coinciding with this cooling interval nearly 20% of the foraminiferal species disappeared while the ammonoid abundance increased. A new and significant warming episode occurred during the latest Aalenian (Limitatum Biochron), extending to the earliest Bajocian (Discites Biochron). Biotic response to this new bT of 2.5 QC is remarkable, marking one of the most important crises of the Aalenian. More than 30% of the foraminiferal species disappeared and no new appearances were recorded. Ammonoids show a decrease in the specimen abundance during the Limitatum-Discites warming, preceding a drastic decrease in the species abundance.The rem...

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

“…7), which has been interpreted as a gradual eutrophication of marine surface waters (Bartolini et al, 1996Bartolini and Cecca, 1999;O'Dogherty et al, 2006;Sandoval et al, 2008;Brigaud et al, 2009;Gómez et al, 2009;Price, 2010). Therefore, this combination of contemporaneous events argues in favour of enhanced productivity of surface waters at a supra-regional scale from the Late Aalenian to the Early Bajocian.…”

“…According to the compilation of the δ 13 O data (Fig. 7) obtained from marine invertebrate and vertebrate remains from the Paris Basin (Brigaud et al, 2009) and Basque-Cantabrian Basin (Gómez et al, 2009), a slight increase in the sea temperatures is recorded between the Concavum (16°-23°C) and Discites zones (15°-25°C). Relatively constant temperatures prevailed from Discites to Laeviuscula zones, even though sample density is weak in this interval.…”

Comparison of benthic foraminifera and geochemical proxies in shelf deposits from the Upper Jurassic of the Prebetic (southern Spain)

“…Yet the influence of oxygen availability on Spirillina, not described in the literature to date, might be considered a limiting factor. Additional factors such as seawater temperature may control the relative abundance of Spirillina (Gómez et al, 2009).…”

Rapid warming and ostracods mass extinction at the Lower Toarcian (Jurassic) of central Spain

“…Therefore, the fossil record offers excellent opportu nities to analyze the strong links between past rapid climate changes and their effects on floral and faunal extinctions (e.g. Kennett and Stott, 1991;Courtillot and Gaudemer, 1996;Culver and Rawson, 2000;Benton and Twitchett, 2003;Mayhew et al, 2008;Gomez et al, 2008Gomez et al, ,2009 and in some cases the records of the lag time needed for a later recovery. et al, 2003, 2004Gomez et al, 2008) and Portugal (Suan et al, 2008).…”