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Malta)Ðcoupling of Weathering Processes and Mid Miocene Climate

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Malta)Ðcoupling of Weathering Processes and Mid Miocene Climate Mixed carbonate-siliciclastic record on the North African margin (Malta)Ðcoupling of weathering processes and mid Miocene climate CeÂdric M. John² Maria Mutti³ Institut fuÈr Geologie und PalaÈontologie, University of Stuttgart, D-70174 Stuttgart, Germany Thierry Adatte Institut de GeÂologie, Universite de NeuchaÃtel, CH-2007 NeuchaÃtel, Switzerland ABSTRACT attached to the North African Margin, we ume since the early Eocene (Miller et al., propose that the observed trends were due 1987; Zachos et al., 2001). Until the early We investigated rock outcrops spanning to a regional increase in rainfall during middle Miocene, ice volume remained low the middle Miocene, global climate-cooling cooler periods, which consequently in- with the exception of several brief periods of step on the Maltese Islands in order to re- creased continental weathering and runoff. glaciations (Mi-events). This episode was fol- construct continental weathering rates and We further suggest that this pattern was lowed by a stepwise but intense reestablish- terrigenous ¯uxes, as well as to explore the linked to the perturbation of atmospheric ment of a major icesheet on Antarctica and coupling between these later, regional cli- fronts due to an increased thermal gradient deep water cooling, as indicated by successive mate and carbonate accumulations. Sedi- during the Miocene. Thus, regional in- d18O shifts beginning at 14.5 Ma (Mi-events mentation at this location was dominated crease in rainfall might have been linked to Mi3a, 3b, 4, Miller et al., 1991a). during the Oligocene and early Miocene by the northward migration of the Intertropi- Mechanisms causing the mid-Miocene cli- a transitional platform to slope carbonates cal Convergence Zone (ITCZ). matic cooling are still a matter of debate. Al- but progressively switched to a clay-rich though it is generally accepted that these cli- carbonate slope system in the middle Mio- Keywords: Miocene, climate, weathering, matic changes were triggered when critical cene. Around 13 Ma, an abrupt change to- clay mineralogy, continental margin, threshold levels of one or more parameters of ward clay-dominated marls occurred, and Malta. the climate system were breached (Kennett marl deposition persisted until the Torton- 1977; Crowley and North, 1988; Zachos, et al. ian (ca. 12 Ma), when a shallow-water car- INTRODUCTION 1993), there is no consensus regarding the re- bonate ramp was reestablished. Clay min- sponsible parameters. Discussion centers eralogy and bulk-rock oxygen isotope The Cenozoic is a time interval character- around two major controls: (1) alteration of analyses suggest that the deposition of the ized by signi®cant changes in climate. The ocean circulation driven by widening or clos- Blue Clay formation was mainly caused by middle Miocene, in particular, is characterized ing of oceanic passages (Kennett, 1977; global climate change and related change in by stepwise cooling and represents a critical Woodruff and Savin, 1989; Wright et al., the rate of continental weathering. period for understanding the establishment of 1992), and (2) pCO2 variability involving CO2 A signi®cant negative correlation (R2 5 the modern climatic system. After this time, sequestering and consequent cooling either via 0.65) exists between the carbonate content the Eastern Antarctic Ice sheet was perma- increased organic carbon burial (Vincent and and the d18O record. This, combined with nently established (Kennett, 1985; Zachos et Berger, 1985; France-Lanord and Derry, 1997) the variation of mass accumulation rate of al., 2001), causing a eustatic sea level drop of or via increased rates of silicate chemical terrigenous material, suggests that shorter- ;100 m (Isern et al., 2001). Ocean circulation weathering (Raymo et al., 1988; Molnar and term periods of globally cooler climate (Mi patterns became similar to the modern ones, England, 1990; Raymo and Ruddiman, 1992). events) were associated with higher rates of and the world globally and permanently cooler These two main controls are often discussed accumulation in continental-derived mate- after the middle Miocene (Kennett, 1985; separately but are not necessarily mutually ex- rial. Since during the Miocene Malta was Flower and Kennett, 1994). clusive (Zachos et al., 2001). The occurrence Glacio-eustatic and carbon cycle changes of a positive shift in carbonate d13C values as ²Present address: University of Potsdam, Earth associated with this transition can be traced much as 1.5½ (Monterey excusion, ;17.5±13 Science Department, P.O. Box 601553, 14415 Pots- using variations in both oxygen and carbon Ma) coeval with the deposition of the organic- dam, Germany; [email protected]. 18 rich Monterey Formation, followed by a pos- ³Present address: University of Potsdam, Earth isotopes. The d O deep-sea record reveals, at Science Department, P.O. Box 601553, 14415 Pots- the long-term scale, a Cenozoic pattern of itive shift in foraminiferal d18O, suggest a pos- dam, Germany. high-latitude cooling and increasing ice vol- sible sequence of increased rates of organic GSA Bulletin; February 2003; v. 115; no. 2; p. 217±229; 12 ®gures; 1 table; Data Repository item 2003032. For permission to copy, contact [email protected] q 2003 Geological Society of America 217 JOHN et al. of strontium for the Miocene (GoddeÂris and FrancËois, 1996) has questioned the siliciclastic weathering hypothesis, and even the notion of a large drop in pCO2 at the mid-Miocene cli- mate shift is currently being argued (Pagani et al., 1999). Thus, understanding the coupling of chang- es in weathering with the carbon cycle is more than ever of direct relevance to the study of these large climatic changes. The goal of the present paper is to explore the relationship and possible feedback between carbonate accu- mulation, continental weathering, and runoff, and to tie these to climate change in the Mio- cene. The Maltese sedimentary record is ideal to study these relationships since it consists of a mixed siliciclastic-carbonate system, offer- ing the opportunity to establish a detailed ox- Figure 1. General map of the Maltese Islands showing the location of Malta, Gozo, and ygen and carbon isotope record based on car- Comino. Stars show the location of the two outcrops (Xatt-L'Ahmar and Qammieh) used bonates and combine it with a clay mineral to build the composite section discussed in this paper. Modi®ed after Felix (1973). record. Most clay minerals inherited from the continents are end products of the siliciclastic weathering chain. Therefore, the study of clay carbon burial, CO removal, and consequent However, no model to date can fully ac- 2 assemblages offers the opportunity to inves- global cooling (Monterey Hypothesis, Vincent count for the observed trends in climate dur- tigate the intensity of continental weathering and Berger 1985). The Monterey excursion ing the Cenozoic. The Monterey Hypothesis as well as the climatic condition under which comprises several higher-frequency (;400 of Vincent and Berger (1985) has been re- these clays were formed. kyr) peaks (carbon maxima, CM, Woodruff cently challenged because the mass accumu- Speci®c objectives of the study are to: (1) and Savin, 1991), which correlate with in- lation rate of organic matter in the Monterey investigate temporal trends in nature and creased benthic foraminifera d18O values and Formation is low, and phosphorus accumula- abundance of clay species in order to track increased carbonate preservation, supporting a tion (thought to be a limiting factor in organic changes in weathering products, (2) recon- relationship between organic carbon burial, matter accumulation) is high even after the struct changes in continental runoff using lower pCO , and bottom water cooling at this major mid-Miocene cooling step (Isaacs, 2 mass accumulation rates of continental de- time (Woodruff and Savin, 1991). 2001; John et al., 2002). Geochemical budget rived materials, and (3) develop a local che- mostratigraphic framework of carbon and ox- ygen isotopes to link the observations made in steps 1 and 2 with global climatic events. GEOGRAPHIC AND LITHOSTRATIGRAPHIC SETTING The Maltese islands are located in the cen- tral Mediterranean and are composed of Malta, Gozo, and Comino (Fig. 1). The Maltese Islands and Sicily were part of the Malta-Ragusa plat- form in the Oligo-Miocene and, as such, at- tached to the North African margin, probably representing a basinward extension of the Tu- nisian carbonate shelf (Dercourt et al., 1993). The lithologic sequence of the Maltese Islands is classically divided into ®ve units (Felix, 1973; see Fig. 2). The lowermost unit is the Lower Coralline Limestone Formation, which consists of mas- sive biogenic limestone beds of shallow ma- rine origin. This shallow carbonate ramp Figure 2. Outcrop picture of the various lithologic units of the Maltese Islands. The Lower phase is Oligocene in age. Deeper water slope Coralline Limestone Formation is under water and, thus, not visible. The Greensand carbonates of the Globigerina Limestone For- Formation forms a very thin interval marked by a white line on the ®gure. FmÐ mation began depositing in the Chattian and Formation. span the early Miocene to late middle Mio- 218 Geological Society of America Bulletin, February 2003 MIXED CARBONATE-SILICICLASTIC RECORD ON THE NORTH AFRICAN MARGIN chlorite content in the Blue Clay formation. The Blue Clay Formation is unconformably overlain by the Greensand Formation and the Upper Coralline Limestone Formation, both late Miocene in age. The Greensand Forma- tion consists of a glauconitic sand bed ranging from 0 to 10 m in thickness, while the Upper Coralline Limestone Formation testi®es to the reestablishment of shallow-water carbonate ramp conditions. COMPOSITE SECTION AND SAMPLING The Globigerina Limestone Formation (ear- ly Miocene to late middle Miocene) and Blue Clay Formation (late middle Miocene to early late Miocene) were investigated since they comprise the time interval of interest to this study. Measuring and sampling were carried out in two locations (Fig. 1) to obtain the most complete and expanded composite section possible (Fig.
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