Delayed Climate Cooling in the Late Eocene Caused by Multiple Impacts: High-Resolution Geochemical Studies at Massignano, Italy
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Earth and Planetary Science Letters 223 (2004) 283–302 www.elsevier.com/locate/epsl Delayed climate cooling in the Late Eocene caused by multiple impacts: high-resolution geochemical studies at Massignano, Italy Bernd Bodiselitscha, Alessandro Montanarib, Christian Koeberl a,b,*, Rodolfo Coccionic a Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria b Osservatorio Geologico di Coldigioco, I-62020 Frontale di Apiro, Italy c Istituto di Geologia e Centro di Palinologia dell’Universita`, Campus Scientifico, Localita` Crocicchia, 61209 Urbino, Italy Received 24 September 2003; received in revised form 6 April 2004; accepted 19 April 2004 Available online 24 June 2004 Abstract High-resolution studies (d13C, d18O, and elemental abundances) were done in rocks at and below the GSSP for the Eocene/ Oligocene (E/O) boundary at Massignano, Italy. In addition to an earlier known Ir anomaly at 5.61 m, which is possibly linked to the Popigai impact event, we confirm the presence of two additional Ir anomalies in the intervals from 6.00 to 6.40 m and from 10.00 to 10.50 m, with maximum values of 259 F 32 ppt at 6.17 m, and 149 F 24 ppt at 10.28 m, respectively. The lower Ir anomaly might be derived from the Chesapeake Bay impact event, whereas for the other one no impact event is known. Similar d13C and d18O trends related to the two Ir anomalies indicate that the Ir anomaly at 10.28 m might be also derived from an impact into a continental shelf, similar to the Chesapeake Bay impact event. d18O values decrease in the high Ir layers to À 1.16x and À 1.17x, respectively, which, together with the negative shifts in d13C in the Ir-rich levels, indicate a warm pulse superimposed on a general Late Eocene cooling trend that is characterized by d18O values ranging between À 0.6x and À 0.4x. The release of methane hydrate after an impact in a continental shelf or seafloor, or impacts of 12C-rich comets during a 2.2-million-year-long comet shower, respectively, could produce these more negative carbon and oxygen excursions compared to the continuously decreasing trend over the whole Late Eocene Massignano section. D 2004 Elsevier B.V. All rights reserved. Keywords: Chesapeake Bay crater; Popigai crater; Late Eocene impact ejecta; Massignano (Italy); global cooling 1. Introduction boundary [3], and significant stepwise floral and faunal turnovers ([1,4,5] and references therein). These The Late Eocene is a period of major changes, global climate changes, which are reflected by a characterized by an accelerated global cooling ([1,2] gradual increase of marine oxygen isotope values and references therein), with a sharp temperature drop (e.g., [6,7]) and biotic crises (e.g., [1,8,9]), are com- of about 2 jC near the Eocene/Oligocene (E/O) monly attributed to the expansion of the Antarctic ice cap following its gradual isolation from other conti- * Corresponding author. Department of Geological Sciences, nental masses [10,11]. However, multiple bolide im- University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria. E-mail addresses: [email protected] pact events ([12] and references therein), possibly (B. Bodiselitsch), [email protected] (A. Montanari), related to a comet shower over a duration of 2.2 [email protected] (C. Koeberl). million years [13,14], may have played an important 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.04.028 284 B. Bodiselitsch et al. / Earth and Planetary Science Letters 223 (2004) 283–302 role related to the deterioration of the global climate at peaks at 5.61 (190 ppt), 6.19 (100 ppt) and 10.25 m (330 the end of the Eocene Epoch. ppt), respectively, in the Massignano section. The age of The two large impact structures Popigai, Russia, and the 5.61 m Ir anomaly was determined at 35.7 F 0.4 Ma, Chesapeake Bay,USA,with respective diametersof100 byinterpolationfromseveraldatedvolcanicashesfound and 85 km, and respective ages of 35.7 F 0.8 Ma [15], in the same section. The overlying peak at 6.19 m is and 35.3 F 0.2 Ma [16–19], represent the largest post- younger by ca. 0.15 Ma. Shocked quartz [35–37], Ni- Cretaceous–Tertiary (K/T) boundary impact events. rich spinels, and microspherules [38] have been found Both have ages near the peak of the Late Eocene comet around 5.61 m, all indicating derivation from an impact shower that was proposed by Farley et al. [14] on the event.Inthelayerthatcontainsshockedquartzat5.61m, basis of a marked enhancement of interplanetary dust no high-pressure silica phases were detected, which are, particle flux in marine sediments. however, present in the Chesapeake Bay related micro- Three smaller impact craters of comparable age, tektite layer of DSDP 612 [39,40]. Langenhorst [37] Mistastin, Canada (38 F 4 Ma, 28 km; [14]), Wanapi- suggested that this shocked quartz was derived from the tei, Canada (37 F 2 Ma, 7.5 km; [21,22]), and Logoisk, nonporous, crystalline target rock at Popigai. That pro- Belarus (40 F 5 Ma, 17 km; [23]), may be part of the posal was recently supported by isotopic data of White- same event, supporting the scenario of a cometary headetal.[33].ExcepttheIranomalyat10.25m,noother bombardment. The comet shower hypothesis predicts evidenceforanimpactwasfoundbyMontanarietal.[34] an even larger occurrence of smaller impacts which at that level. may have played a role in the alteration climate con- To determine whether these peaks are associated ditions at a global scale, due to atmospheric blowout with an impact event or not, we checked Ir/Fe ratios; and distribution of ejecta around the Earth [24]. possible volcanic input is discussed using trace element At least two distinct, yet closely spaced, Late Eocene ratios. To investigate the influence of possible impact impact spherule layers, the older containing microtek- events on seawater temperature we analyzed oxygen tites, and the younger microkrystites [25,26], have been and carbon isotopes from bulk-carbonates. It is well identified in ocean sediments from the Atlantic, Indian known that weathering and/or diagenesis may affect and Pacific Oceans, the Caribbean Sea and the Weddell the original isotopic composition. In the case of the Sea, off Antarctica [27]. These distal impact ejecta were Massignano section, being located on a relatively fresh also found in Late Eocene sediments in Texas, Georgia, quarry cut, there are no indications of recent weather- Massachusetts, Barbados and Cuba [8,28–31].The ing, and the section also is not disturbed by tectonics, North American tektites were proposed to be derived lacking faulting or folding. No recrystallization of the from the Chesapeake Bay impact event [17,18,32], carbonate phases were found, indicating that the car- whereastheclinopyroxene-bearingspherulestrewnfield bonate phase is not diagenetically modified [41]. How- (i.e., the microkrystites) may be linked to the Popigai ever, Vonhof et al. [42] have noted, from SEM analysis, crater [15,33]. Estimates of the time separating the two that the foraminifers in the Massignano section are layersrangefrom10to20ky[25],and3to5ky[12],with filled with secondary, blocky calcite. theChesapeakeBayimpactbeingtheyoungerevent.The In absence of well-preserved, species-determined clinopyroxene-spherule layer contains an Ir anomaly, calcite tests or shells, and under certain diagenetic shocked quartz, Ni-rich spinels, and impact spherules. conditions, bulk rock calcite may represent an accept- Here we present the results of the chemical composi- able material for stable isotope analysis, particularly for tion, and oxygen and carbon isotope ratios of pelagic carbon [43,44]. Even compacted and cemented car- marly limestones sampled in the Massignano section bonate pelagic sediments may retain the original d13C (central Italy), which represents the Global Stratotype signal. On the other hand, oxygen isotopic ratios in SectionandPoint(GSSP)fortheE/Oboundary.Samples bulk-rock samples are generally much more suscepti- frommeterlevel6.00–6.40and10.00–10.50oftheLate ble to alteration during diagenesis than carbon isotopes Eocenewerestudied.Alsotheoxygenandcarbonisotope [45]. Oxygen isotopic fractionation is more affected by variations over the whole Massignano section from 0 m temperature during recrystallization than carbon. (Late Eocene) to 23 m (Early Oligocene) were investi- For the determination of the paleoceanographic and gatedindetail.Formerstudiesby[34]showprominent Ir paleoclimatic conditions, the usage of foraminiferal B. Bodiselitsch et al. / Earth and Planetary Science Letters 223 (2004) 283–302 285 species is more common than bulk-carbonate analysis. this isotope ratio may reflect variations in d13C contents Bulk-carbonate samples represent a mixture of carbo- of ocean water [48]. A higher 13C/12C ratio can be nates from different sources, e.g., benthonic and plank- interpreted as a decrease in bio-productivity resulting in tonic foraminifers and calcareous nannofossils. The a decrease in organic matter accumulation in the isotopic composition of bulk samples is a function of sedimentary record. This can also be interpreted as the composition of these species—e.g., d18O of fora- the consequence of a cooling event. Thus, we used the minifers—is a function of the seawater d18O value d13C values, together with the d18O values, to derive where organism lived. Environmental changes could cooling or warming trends. be established if these changes have an effect on most of the species. Under these circumstances, the isotopic composition derived from bulk analyses resembles 2. Location and stratigraphic documentation closely the record derived from single foraminiferal analyses [46,47]. The abandoned quarry of Massignano is located Thus, d18O values could be used to infer changes in along the provincial road of the Monte Co`nero Park, water temperature through a given stratigraphic inter- about 4 km north of the town of Sirolo (Fig.