Earth and Planetary Science Letters, 118 (1993) 145-166 145 Elsevier Science Publishers B.V., Amsterdam [vdVl Magnetostratigraphic calibration of the Late Valanginian carbon isotope event in pelagic limestones from Northern Italy and Switzerland J.E.T. Channell, E. Erba and A. Lini a Department of Geology, University of Florida, Gainesville, FL 32611, USA b Dipartimento di Seienze della Terra, Universith di Milano, 20133 Milan, Italy c Geologisches lnstitut, ETH Zentrum, 8092 Ziirich, Switzerland Received January 7, 1993; revision accepted May 24, 1993 ABSTRACT Magnetostratigraphic, biostratigraphic and carbon isotope data are presented from five Lower Cretaceous pelagic limestone sections in the Southern Alps. A positive carbon isotope event is recorded in bulk carbonate from the Upper Valanginian of all five sections, and the magnetostratigraphy and biostratigraphy provide the age model. The record of the isotope event in absolute time is closely replicated in all five sections. As the sections are located in two paleogeographic basins and are separated by up to 300 km, the synchronous isotope event cannot be interpreted as an artifact of diagenesis. A reduction in magnetization intensity coincident with the 613C peak may be due to diagenetic magnetite dissolution enhanced by increased organic carbon burial. According to the age model, which uses GTS89 timescale [1], background ~13C values of 1.5%o are recorded from 145 to 137 Ma. Between 137 and 136 Ma, ~13C increases to a maximum value of about 3%o. Values begin to decrease from 136 Ma and reach background values at about 132 Ma. The rate of change of 613C for the onset of the event (about 1.5%o/m.y.) is greater than for the decay (about 0.5%o/m.y.), suggesting an abrupt perturbation of the carbon system from equilibrium, followed by a more gradual return to background values. 1. Introduction sections [8,9] and deep-sea cores [10]. As in previ- ous papers [3-5], polarity chrons are numbered The correlation of Early Cretaceous calcare- according to the correlative M-sequence oceanic ous nannofossil events to polarity chrons has been anomaly, with the prefix 'C' to distinguish the established by replicating this correlation in nu- chron (time interval) from the oceanic anomaly. merous Italian pelagic limestone sections [2-5]. Numbered oceanic magnetic anomalies usually The combination of magnetostratigraphy and correlate to reversed polarity chrons. Intervening nannofossil biostratigraphy now provides a practi- normal polarity chrons are given the number of cal integrated stratigraphic tool for Lower Creta- the next (older) reveresed chron with 'n' ap- ceous pelagic limestones in Italy, and similar sed- pended. iments of this age elsewhere [6,7]. The nannofos- Carbon isotope geochemistry provides a tracer sil events aid the identification of polarity chrons which can be used to monitor past global environ- and the resulting magnetostratigraphy provides mental conditions [11-18]. The Cretaceous is the detailed stratigraphic control not available characterized by a number of positive carbon from nannofossil biostratigraphy alone. In this isotope events observed in whole-rock studies of paper, the integrated stratigraphic approach is pelagic limestones [11,12,18]. The best studied used to evaluate the synchroneity and timing of positive carbon isotope events in the Cretaceous the Late Valanginian carbon isotope event, which time interval are at the Cenomanian-Turonian has been observed in Italian pelagic limestone boundary [13-15], in the Aptian [8,11,16] and in 0012-821X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved 146 J.E.T. CHANNELL, E. ERBA AND A. LINI the Late Valanginian [9]. Another well-studied have led to elevated transfer rates of nutrients to carbon isotope event occurs during the mid- the oceans, favoring primary productivity and en- Miocene [19]. These globally recognized 613C hancing rates of biological carbon storage in the anomalies, which reflect repeated perturbations oceans. For the deposition of the mid-Cretaceous of the global carbon cycle, have duration of a few rhythmic black shales, it is generally more popu- million years and peak 613C values 1-2%o above lar to invoke oceanic stratification and poor pre-excursion values. oceanic circulation (linked to sea-level rise or Carbon isotope events generally coincide with climate change) as the primary cause for the stratigraphic evidence for increased organic car- enhanced preservation of organic carbon [22-24]. bon burial, although this coincidence is more According to the model proposed by Schlanger subtle for the Late Valanginian event. The pres- and Jenkyns [25] and Arthur et al. [14] for the ence of black shales in the Aptian and at the Cenomanian-Turonian carbon isotope event, a Cenomanian-Turonian boundary has led to these major rise in global sea level resulted in flooding events being referred to as 'oceanic anoxic events' of expanded shelf areas, leading to increased in the Cretaceous of the Atlantic and Tethyan surface water productivity and expansion of the realms. Fluctuations in the carbon isotope record mid-water oxygen-minimum zone which favored can be explained by varying export rates of or- organic carbon preservation. These authors ganic carbon from the atmospheric and oceanic therefore invoked sea-level fluctuations as the reservoirs to the sedimentary reservoir. Positive driving force for changing organic carbon burial 613C events result from an increase in the burial rates. Models involving marine transgressions ratio of organic carbon to carbonate in marine causing changes in surface productivity and burial sediments. The trigger for these short (a few rate of organic carbon on shallow continental million years) carbon burial events remains enig- shelves have also been proposed to explain the matic. The major positive ~13C events mentioned mid-Miocene positive carbon isotope excursion above seem to coincide with times of increased [19]. deposition of phosphate-rich sediments (e.g., the The Late Valanginian carbon isotope event mid-Miocene Monterey Formation), thus suggest- can be calibrated using magnetostratigraphy, ing a coupling between anomalies in the carbon whereas such age control is not available for the and phosphorous cycles [20]. Within available age Aptian and Cenomanian-Turonian 313C events constraints, the 313C events appear to correlate which are within the Cretaceous long normal with sea-level highstands and increased global polarity interval. In this paper, we present new volcanic activity. isotope and magneto- and biostratigraphies from A number of mechanisms, such as climate three Majolica sections (Breggia, Pusiano and warming, increased biogenic productivity and Val del Mis). We use previously published iso- changes in oceanic circulation patterns (e.g., tope and magneto- and biostratigraphies from oceanic stratification) have been invoked to link Polaveno [5,9] and Capriolo [3,9] together with volcanic activity and/or sea-level rise to viable additional isotope data from these two localities. triggers for enhanced organic carbon burial and The magnetostratigraphies are used to produce the observed 613C excursions. Some authors cite age models for the isotope event at all five sec- increased primary productivity stimulated by cli- tions. Comparison of the isotope anomaly in dif- mate change as the primary cause of increased ferent sections and assessment of rates of change carbon burial. The Aptian and Late Valanginian of 613C may provide constraints on genetic mod- ~3C events have been postulated as being due to els for Cretaceous carbon isotope/burial events. accelerated carbon cycling triggered by elevated atmospheric CO 2 levels coupled with a warm and 2. Geological setting humid climate [8,9]. These episodes of green- house climate conditions may be related to in- The Mesozoic continental margin sediments creased CO 2 emissions caused by intensified exposed in the Southern Alps record the Jurassic volcanic activity [21]. Enhanced continental and Cretaceous subsidence history of this part of weathering combined with increased runoff may the southern Tethyan continental margin since its MAGNETOSTRATIGRAPHIC CALIBRATION OF THE VALANGINIAN CARBON ISOTOPE EVENT, N. ITALY AND SWITZERLAND 147 Liassic inception [26]. The Tithonian to Aptian biostratigraphic framework in the Maiolica For- interval is represented by the Maiolica Forma- mation is derived from calcareous nannofossils, tion, which is a regularly bedded white/gray with additional control from calpionnellids in the cherty pelagic limestone [27,28]. The formation is Tithonian-Valanginian, and planktonic foramini- well exposed throughout the Southern Alps fera in the Barremian-Aptian. (Northern Italy) and the Umbria-Marche Basin Of the five Maiolica sections discussed in this (Central Italy). Similar coeval facies have been paper, four are located in the Lombardian Basin drilled in the Atlantic and Pacific oceans. Al- and one in the Belluno Basin (Fig. 1). All five though the Maiolica in the Southern Alps shows sections are in Italy, apart from the Breggia sec- rather abrupt thickness variations, from a few tion which is located in Switzerland, a few kilo- meters to over 300 m, the formation blankets meters from the Italian border. Previously pub- both the structural high (Trento Plateau) and the lished section descriptions are available for Ca- intervening basins (Lombardian and Belluno priolo [3], Polaveno [5,29] and part of Breggia [9]. basins) which characterized this part