One of the main controversial themes in palaeoclimatology involves
elucidating whether climate during the Jurassic was warmer than the present
day and if it was the same over Pangaea, with no major latitudinal gradients.
There has been an abundance of evidence of oscillations in seawater temperature
throughout the Jurassic. The Pliensbachian (Early Jurassic) constitutes a
distinctive time interval for which several seawater temperature
oscillations, including an exceptional cooling event, have been documented.
To constrain the timing and magnitude of these climate changes, the Rodiles
section of the Asturian Basin (Northern Spain), a well exposed succession of
the uppermost Sinemurian, Pliensbachian and Lower Toarcian deposits, has
been studied. A total of 562 beds were measured and sampled for ammonites,
for biochronostratigraphical purposes, and for belemnites, to determine the
palaeoclimatic evolution through stable isotope studies. Comparison of the
recorded latest Sinemurian, Pliensbachian and Early Toarcian changes in
seawater palaeotemperature with other European sections allows
characterization of several climatic changes that are likely of a global
extent. A warming interval partly coinciding with a
Location maps of the Rodiles section.
The idea of an equable Jurassic greenhouse climate, 5–10
Especially relevant are the latest Pliensbachian
Nevertheless, with the exception of several sections (Rosales et al., 2004; Korte and Hesselbo, 2011; Suan et al., 2008, 2010), few data have been published on the evolution of seawater palaeotemperatures during the latest Sinemurian and the Pliensbachian, even some more papers studied the climatic changes of parts of the Late Pliensbachian and Early Toarcian (i.e. McArthur et al., 2000; Hesselbo et al., 2000; Jenkyns et al., 2002; van de Schootbrugge et al., 2010; Gómez and Goy, 2011; Armendáriz et al., 2012; Harazim et al., 2013).
The present paper attempts to provide data on the evolution of seawater palaeotemperatures and on changes in carbon isotopes through the Late Sinemurian, Pliensbachian and Early Toarcian (Early Jurassic) and to constrain the timing of the recorded changes through ammonite-based biochronostratigraphy. The data set was obtained from the particularly well-exposed Rodiles section, located in the Asturia's regional autonomy in Northern Spain (Fig. 1). Our results have been correlated with the records obtained in different sections of Europe, showing that these climatic changes, as well as the documented perturbations of the carbon cycle, could be of global, or at least of regional extent at European scale.
In the coastal cliffs located northeast of the Villaviciosa village, in the
eastern part of the Asturias regional autonomy (Northern Spain) (Fig. 1),
the well exposed Upper Sinemurian, Pliensbachian and Lower Toarcian deposits
are represented by a succession of alternating lime mudstone to bioclastic
wackestone and marls with interbedded black shales belonging to the Santa
Mera Member of the Rodiles Formation (Valenzuela, 1988) (Fig. 2). The
uppermost Sinemurian and Pliensbachian deposits were studied in the eastern
part of the Rodiles Cape and the uppermost Pliensbachian and Lower Toarcian
in the western part of the Rodiles Cape (West Rodiles section of Gómez
et al., 2008; Gómez and Goy, 2011). Both fragments of the section are
referred to here as the Rodiles section (lat. 43
Sketch of the stratigraphical succession of the uppermost Triassic
and the Jurassic deposits of the Asturian Basin. The studied interval
corresponds to the lower part of the Santa Mera Member of the Rodiles
Formation. Pli.
The 110 m thick section studied, comprising 562 beds, was studied bed by bed. Collected ammonites were prepared and studied following the habitual palaeontological methods (Comas-Rengifo, 1985; Phelps, 1985; Howarth, 2002). The biochronostratigraphy obtained enabled characterization of the standard chronozones and subchronozones established by Elmi et al. (1997) and Page (2003), which are used in the present research.
A total of 191 analyses of stable isotopes were performed on 163 belemnite
calcite samples, in order to obtain the primary Late Sinemurian,
Pliensbachian and Early Toarcian seawater stable isotope signal, and hence
to determine palaeotemperature changes, as well as the variation pattern of
the carbon isotope in the studied time interval. In order to assess possible
burial diagenetic alteration of the belemnites, polished samples and thick
sections of each belemnite rostrum were prepared. The thick sections were
studied under the petrographic and the cathodoluminescence microscope, and
only the non-luminescent, diagenetically unaltered portions of the belemnite
rostrum were sampled using a microscope-mounted dental drill. Sampling of
the luminescent parts such as the apical line and the outer and inner
rostrum wall, fractures, stylolites and borings were avoided. Belemnite
calcite was processed in the stable isotope labs of Michigan University
(USA), with a Finnigan MAT 253 triple collector isotope ratio mass
spectrometer. The procedure followed in the stable isotope analysis has been
described in Gómez and Goy (2011). Isotope ratios are reported in per
mil relative to the standard Peedee belemnite (PDB), presenting a
reproducibility better than 0.02 ‰ PDB for
The seawater palaeotemperature recorded in the oxygen isotopes of the
belemnite rostra studied have been calculated using the Anderson and Arthur (1983) equation:
To calculate palaeotemperature, it has been assumed that the
Discussion of the palaeoecology of belemnites, or the validity of the isotopic data obtained from belemnite calcite for the calculation of palaeotemperatures do not fall within the scope of this research, but the use of belemnite calcite as a proxy is generally accepted and widely used as a reliable tool for palaeothermometry in most of the Mesozoic. However, belemnite palaeoecology constitutes a source of conflicts because, due to the fact that these organisms are extinct, there is a complete lack of understanding of fossil belemnite ecology (Rexfort and Mutterlose, 2009). Belemnites lived as active predators within swimming life habitats. Nevertheless, several authors (Anderson et al., 1994; Mitchell, 2005; Wierzbowski and Joachimiski, 2007) have proposed a bottom-dwelling lifestyle on the basis of oxygen isotope thermometry, similar to modern sepiids which show a nektobenthic mode of life. This is contradicted by the occurrence of various belemnite genera in black shales which lack any benthic or nektobenthic organisms due to the existence of anoxic bottom waters (i.e. the Lower Jurassic Posidonienschiefer, see Rexfort and Mutterlose, 2009), a fact that indicates that belemnites presented a nektonic mode of life rather than a nektobenthic (Mutterlose et al., 2010). As Rexfort and Mutterlose (2009) stated, it is unclear whether isotopic data from belemnites reflect a surface or a deeper water signal, and we are unaware whether the belemnites mode of life changed during ontogeny. Similarly, Li et al. (2012) concluded that belemnites were mobile and experienced a range of environmental conditions during growth; furthermore, these authors stated that some belemnite species inhabited environmental niches that remain unchanged, while other species had a more cosmopolitan lifestyle inhabiting wider environments. To complete the scenario, Mutterlose et al. (2010) suggested different lifestyles (nektonic versus nektobenthic) of belemnite genera as indicated by different shaped guards. Short, thick guards could indicate nektobentic lifestyle, elongated forms fast swimmers, and extremely flattened guards a benthic lifestyle.
The study by Ullmann et al. (2014) hypothesizes that belemnites
(
The isotopic studies performed on present-day cuttlefish (
It seems that at least some belemnites could swim through the water column, reflecting average temperature and not necessarily only bottom or surface water temperatures. In any case, rather than single specific values, in the present paper comparisons of average temperatures to define the different episodes of temperature changes are used.
Ammonite taxa distribution and profiles of the
The Upper Sinemurian, Pliensbachian and Lower Toarcian deposits of the
Rodiles section comprise couplets of bioclastic lime mudstone to wackestone
limestone and marls. These limestones occasionally contain bioclastic
packstone facies concentrated in rills. Limestones, generally recrystallized
to microsparite, are commonly well stratified in beds whose continuity can
be followed at the outcrop scale, as well as in outcrops several kilometres
apart. However, nodular limestone layers, discontinuous at the outcrop
scale, are also present. The base of some carbonates can be slightly
erosive, and they are commonly bioturbated, to reach the homogenization
stage. Ichnofossils, especially
The ammonite-based biochronostratigraphy of these deposits in Asturias was performed by Suárez-Vega (1974), and the uppermost Pliensbachian and Toarcian ammonites by Gómez et al. (2008) and by Goy et al. (2010a, b). Preliminary biochronostratigraphy of the Late Sinemurian and the Pliensbachian in some sections of the Asturian Basin has been reported by Comas-Rengifo and Goy (2010), and herein we summarize the result of over 10 years of bed by bed sampling of ammonites in the Rodiles section, which provided a precise time constraint for the climatic events described in this work.
Stratigraphical succession of the Upper Sinemurian, the
Pliensbachian and the Lower Toarcian deposits of the Rodiles section,
showing the lithological succession, the ammonite taxa distribution, as well
as the profiles of the
The ammonites collected enabled recognition of all the standard Late Sinemurian, Pliensbachian and Early Toarcian chronozones and subchronozones defined by Elmi et al. (1997) and Page (2003) for Europe. The section is generally expanded and ammonites are sufficiently common to constrain the boundaries of the biochronostratigraphical units. Exceptions are the Taylori-Polymorphus subchronozones that could not be separated, and the Capricornus-Figulinum subchronozones of the Davoei Chronozone, partly due to the relatively condensed character of this Chronozone. Most of the recorded species belong to the NW Europe province but some representatives of the Tethysian Realm are also present.
Thick sections photomicrographs of some of the belemnites sampled
for stable isotope analysis from the Upper Sinemurian and Pliensbachian of
the Rodiles section. The unaltered by diagenesis non-luminescent sampling
areas (SA), where the samples have been collected, are indicated.
Belemnites in the Rodiles section generally show an excellent degree of preservation (Fig. 4) and none of the prepared samples were rejected, as only the non-luminescent parts of the belemnite rostrum not affected by diagenesis were selected. It has been assumed that the biogenic calcite retains the primary isotopic composition of the seawater and that the belemnite migration, skeletal growth, sampling bias, and vital effects are not the main factors responsible for the variations obtained.
The cross-plot of the
Cross-plot of the
The carbon isotopes curve reflects several oscillations throughout the
section studied (Fig. 3). A positive
At the Late Pliensbachian the
The
The isotope curves obtained in the Upper Sinemurian, Pliensbachian and Lower Toarcian section of the Asturian Basin have been correlated with other successions of a similar age, in order to evaluate whether the environmental features recorded present a local or possible global extent. In order to correlate a more homogeneous data set, we only employed the isotopic results obtained by other authors from belemnite calcite and exceptionally from brachiopod calcite.
The detailed biostratigraphical analysis, based on the succession of the Pliensbachian ammonoids assemblages allowed construction of a scale of reference that has facilitated the location of the different palaeoclimatic events recognized in the present research.
The five biochronozones of the standard scale constituting the Pliensbachian of the Subboreal/NW Europe Province (Dommergues et al., 1997; Page, 2003) have been recognized in the Rodiles section. For the first time, these biochronozones have been subdivided into 14 subchronozones whose boundaries have been corrected in many cases with respect to previous studies. In most cases these boundaries have now been established with a low margin of uncertainty.
With regard to previous research (Suárez-Vega, 1974; Comas-Rengifo and Goy, 2010) the Taylori and Brevispina subchronozones of the Early Pliensbachian have been characterized in this study for the first time, and the boundary between the Valdani and the Luridum subchronozones, usually difficult to distinguish in the Asturian Basin, has been clearly recognized. In the Late Pliesbachian, where the record of Amaltheidae is quite complete, the subchronozone Apyrenum of the Spinatum Chronozone has been characterized and the boundary between the Subnodosus and Gibbosus subchronozones has been precisely established.
Correlation chart of the belemnite calcite-based
The
The Early Pliensbachian
Korte and Hesselbo (2011) pointed out that the Early Pliensbachian
Higher in the section, the
The next CIE involves a positive excursion of around
1.5–2 ‰, well recorded in all the correlated Upper
Pliensbachian sections (the Late Pliensbachian positive excursion in Fig. 6)
and in bulk carbonates of the Lusitanian Basin (Silva et al., 2011; Silva
and Duarte, 2015) and in the Apennines of Central Italy (Moretinni et al.,
2002). This CIE also partly coincides with the
Finally, the Early Toarcian is characterized by a prominent
The origin of the positive excursion has been interpreted by some authors as
the response of water masses to excess and rapid burial of large amounts of
organic carbon rich in
Although
The origin of the Early Toarcian
Martinez and Dera (2015) proposed the presence of fluctuations in the carbon
cycle during the Jurassic and Early Cretaceous, resulting from a cyclicity
of
Curve of seawater palaeotemperatures of the Late Sinemurian, Pliensbachian and Early Toarcian, obtained from belemnite calcite in the Rodiles section of Northern Spain. Two warming intervals corresponding to the Late Sinemurian and the Early Pliensbachian are followed by an important cooling interval, developed at the Late Pliensbachian, as well as a (super)warming event recorded in the Early Toarcian. Chronozones abbreviations: RAR: Raricostatum. D: Davoei. TENUICOSTA.: Tenuicostatum. Subchronozones abbreviations: DS: Densinodulum. RA: Raricostatum. MC: Macdonnelli. AP: Aplanatum. BR: Bevispina. JA: Jamesoni. VA: Valdani. LU: Luridum. CA: Capricornus. FI: Figulinum. SU: Subnodosus. PA: Paltum. SE: Semicelatum. FA: Falciferum.
Seawater palaeotemperature calculation from the
Correlation chart of the belemnite calcite-based
The earliest isotopic event is a
The Late Sinemurian Warming interval is also recorded in the Cleveland Basin
in the UK (Hesselbo et al., 2000; Korte and Hesselbo, 2011). The
belemnite-based
The Late Sinemurian warming coincides only partly with the Early
Pliensbachian
Following the Late Sinemurian Warming,
Most of the Early Pliensbachian Ibex Chronozone and the base of the Late
Pliensbachian are dominated by a negative excursion ranging from 1 to
1.5 ‰
The Early Pliensbachian Warming interval is also well marked in other
sections of Northern Spain (Fig. 8) such as the Asturian Basin
(Armendáriz et al., 2012) and the Basque–Cantabrian Basin (Rosales et
al., 2004), where peak values of around 25
One of the most important Jurassic
This major cooling event has been recorded in many parts of the World. In Europe, the onset and the end of the cooling interval would appear to be synchronous at the scale of the ammonites subchronozone (Fig. 8). It starts in the Stokesi Subchronozone of the Margaritatus Chronozone (near the onset of the Late Pliensbachian), and extends up to the Early Toarcian Semicelatum Subchronozone of the Tenuicostatum Chronozone. In addition to the Asturian Basin (Gómez et al., 2008; Gómez and Goy, 2011; present paper), it has clearly been recorded in the Basque–Cantabrian Basin (Rosales et al., 2004; Gómez and Goy, 2011; García Joral et al., 2011) and in the Iberian Basin of Central Spain (Gómez et al., 2008; Gómez and Arias, 2010; Gómez and Goy, 2011), in the Cleveland Basin of the UK (McArthur et al., 2000; Korte and Hesselbo, 2011), in the Lusitanian Basin (Suan et al., 2008, 2010), in the French Grand Causses Basin (van de Schootbrugge et al., 2010), and in the data compiled by Dera et al. (2009, 2011).
As for many of the major cooling periods recorded in the Phanerozoic, low
levels of atmospheric
The Late Pliensbachian appears to represent a time interval of major cooling, likely at global scale. This is why many authors point to this period as one of the main candidates for the development of polar ice caps in the Mesozoic (Price, 1999; Guex et al., 2001; Dera et al., 2011; Suan et al., 2011; Gómez and Goy, 2011; Fraguas et al., 2012). This idea is based on the presence, in the Upper Pliensbachian deposits of different parts of the World, of (1) glendonites; (2) exotic pebble to boulder-size clasts; (3) the presence in some localities of a hiatus in the Late Pliensbachian-earliest Toarcian; (4) the results obtained in the General Circulation Models, and (5) the Late Pliensbachian palaeotemperatures calculated and the assumed pole-to-equator temperature gradient.
Seawater temperature started to increase in the earliest Toarcian. From an
average temperature of 12.7
Several relevant climatic oscillations across the Late Sinemurian, the
Pliensbachian and the Early Toarcian have been documented in the Asturian
Basin. The correlation of these climatic changes with other European records
indicates that some of these might be at global scale. In the Late
Sinemurian, a warm interval showing an average temperature of
18.5
The Late Sinemurian Warming interval is followed by a period of temperature
averaging 16
The latest part of the Early Pliensbachian is dominated by an increase in
temperature, marking another warming interval which extends to the base of
the Late Pliensbachian, where an average temperature of 18.2
One of the most important climatic changes was recorded throughout the Late
Pliensbachian. An average palaeotemperature of 12.7
Seawater temperature started to increase in the earliest Toarcian, rising to
15
We thank three anonymous reviewers and the editor for their comments and suggestions that improved the manuscript. This research work was financed by project CGL2015-66604-R of the Spanish Ministerio de Economía y Competitividad, and by projects GR3/14/910431, and GI 910429 of the Universidad Complutense de Madrid. Thanks to the Instituto Geológico y Minero de España for allowing the use of the cathodoluminescence microscope. Edited by: A. Sluijs