Publication: Palaeoclimatic and biotic changes during the Aalenian (Middle Jurassic) at the
southern Laurasian Seaway (Basque–Cantabrian Basin, northern Spain)
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2009
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Elsevier
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Abstract
The uppermost Toarcian–lowermost Bajocian 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 O isotope.
Facies and thickness distribution of the Aalenian sediments suggest that deposition took place in a nearly
symmetrical sub-basin included into an intraplate shallow platform, developed over continental crust, on
which flexure was the main control responsible for subsidence and basin evolution.
Noteworthy negative δ13Cbel 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 δ18Obel-based palaeotemperature shows a remarkable cooling interval with an
average seawater temperature of 15.7 °C. 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 °C, 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 ΔT
of 2.5 °C 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 remarkable biotic changes linked to the variations of seawater temperature recorded in the Basque–
Cantabrian Basin during the Aalenian, open the possibility that some of these faunal turnovers, specially
marked in the benthic foraminifers, were of global extent and mainly forced by climate changes.