The SCPFB hosts a foreland Early Paleogene succession displaying many features that suggest an interaction of both tectonic and climatic controls over the sedimentary succession (Mutti & al, 1994). According to the latter paper, in some parts of the succession it may still be difficult to clearly recognize the effective roles of the two factors, and a great number of stratigraphic studies have been performed to define the sequence stratigraphic framework of the succession (see geologic framework).
As previously stated, the study of stable isotopes allows global correlation of major climatic trends and events; the Early Paleogene is probably the oldest thoroughly investigated stratigraphic interval in terms of chronology of these events.
In this work we consider both the chronostratigraphic significance of the hyperthermals, since these events develop and recede globally on relatively short amounts of time (tens to few hundreds ka), and their long-term climatic variations that may be put in relation with the succession.
Former isotopic studies performed in the study area followed similar purposes; examples are the study of the K/Pg boundary in the Ager basin (López-Martínez et al, 1996), or the detection of the PETM along the northern sector of Tremp basin (Schmitz & Pujalte, 2003, 2007), or the isotopic characterization of the correlative “Campo Section” westward of the study area (Molina et al, 2003).
The analyses performed for this work were undertaken on a variety of lithologies, and although a temporal series of such heterogeneous data is in itself poorly significant, the characterization in age and lithology is shown in carbon/oxygen diagrams, so that vertical successions can be correctly interpreted.
Stable isotopes of carbon and oxygen were studied in marine and continental carbonates, and a tentative study of organic matter carbon stable isotopes was performed in the Paleocene of Ager basin. As it is made evident by some samples, oxygen isotopes in carbonates turned out to be the least preserved signal. In some cases facies and isotopic analyses coupled allowed the recognition of diagenetic horizons (such as the negativization trend produced by the subaerial exposure of late-Thanetian marine carbonates in Ager basin); and finally, local environmental effects on isotopic compositions were observed (such as the positive values recorded in sabkha carbonates in the upper
Thanetian of Ager Basin).
5.3.1 – Materials and Methods
Sample preparation and analysis were carriedout at the Isotopic Geochemistry Laboratory at the Università degli Studi di Parma Earth Sciences Department.
5.3.1.1 – Carbonates
SAMPLE COLLECTION
Carbonate samples were collected as hand samples always trying to get unaltered, non-surficial samples; carbonate soil nodule samples were collected individually by digging a few tens of centimeters in the hosting paleosol.
Carbonate samples were generally sub-sampled with a dentist drill, unless individual samples turned out to be too small (e.g., the carbonate soil nodules were generally split and drilled in the middle, with the exception of some nodules from section [14], that were individually treated as bulk because of their small size – in any case, no bulking of multiple nodules was performed).
Continental limestones (commonly micrites) were sub-sampled in order to subdivide original micrite, burrow fills, altered horizons (most of the times related with Microcodium bioerosion) and any other macroscopic feature.
Marine limestones were most of the times treated as bulk samples, in the absence of a microsampling device, with the exception of a few samples where a presumably original micritic matrix was abundant enough to be drilled separately from the rest of the sample.
Almost all of the carbonates analyzed were close to pure, low-Mg limestones; exceptions are discussed below.
PREPARATION AND ANALYSIS
The powders obtained from drilling, and subsequent powdering in an agate mortar, were commonly weighed between 6.0 and 12.0 mg, then prepared with a manual vacuum line, through which they were reacted with excess 100% phosphoric acid at ~5*10-3 atm and 25°C for at least 12 hours.
A few samples, containing a significant dolomitic component (see 5.4.1.3), underwent a
two-steps procedure in order to separate carbon dioxide produced by calcite or dolomite reaction:
samples were weighed between 20.0 and 40.0 mg, reacted with 100% phosphoric acid, and carbon dioxide produced by the fast reaction of calcite was extracted for spectrometer analysis after two hours; the reaction was then continued for two more hours and further products of the reaction from calcite were totally removed. Finally, after at least 72 hours of reaction, carbon dioxide freed from dolomite reaction was extracted and analyzed.
Carbon dioxide analysis was performed with a Thermo Finnigan Delta S mass spectrometer by 8 comparisons with an internal standard (MAB99); data were then expressed in permille with reference to the V-PDB international standard.
About 25% of the samples were double analyzed, with an analytical plus instrumental error summing up to ±0,2‰.
5.3.1.2 – Organic matter
One of the main purposes of the isotopic study in Ager basin Paleocene was the correlation of the thinned northern succession with the thick southern one; because of the scarcity of pedogenic nodules in the latter, a tentative survey on organic matter carbon isotopes was performed.
Similar studies were formerly performed in Early Paleogene continental settings (Domingo & al 2009) for the PETM interval in Tremp basin as well as in other locations all over the world (e.g., Magioncalda et al, 2004), showing the coupling of the inorganic (carbonate) and organic matter carbon isotopic signal.
SAMPLE COLLECTION
Carbonate paleosols were collected preferentially at some tens of centimeters from the surface, if possible avoiding internal discontinuities such as slickensides, and in the absence of modern vegetation.
Ager basin Paleocene southern section (section [4]) was sampled in detail in its lower and upper portions, while in its middle portion the presence of a stronger plant cover hampered the possibility to collect good samples for the purposes of this survey.
PREPARATION AND ANALYSIS
Paleosol samples were dried at low temperature (~30°C), then powdered with an agate mortar
and successively reacted with 25% HCl for 60 minutes, while manually kept in suspension. Since samples showed a CaCO3 content between 20% and 50%, about 3 ml of acid were used for each gram of sample. After the decarbonation, samples were neutralized using a centrifuge; during this process, the organic matter differentiates inside the test tube as a thin black layer on top of the clay.
Since the amount of organic matter cannot be clearly quantified in this phase, decarbonated samples are dried again at low temperature, and again powdered and homogenized using the agate mortar.
The organic carbon content and its isotopic composition were estabilished in double analysis with a coupled elemental analyzer/mass spectrometer (analyses were performed by A. di Matteo at the Isotopic Geochemistry Laboratory at the Università degli Studi di Parma Earth Sciences Department) after weighing of the samples between 20 and 40 mg, in function of their abundance in organic matter.
PREMISE FOR DATA EXPOSITION
All ∂18O and ∂13C data are expressed in ‰ vs V-PDB; if not explicit, terms in the text such as
“negative” or “positive” are to be intended with respect to this standard.