Downloaded from geology.gsapubs.org on July 21, 2015 Short-term euxinia coinciding with rotaliporid extinctions during the Cenomanian-Turonian transition in the middle-neritic eastern North Atlantic inferred from organic compounds Masahiro Oba1*, Kunio Kaiho1, Takashi Okabe1, Marcos A. Lamolda2, and James D. Wright3 1Institute of Geology and Paleontology, Tohoku University, Sendai 980-8578, Japan 2Departamento de Estratigrafía y Paleontología, Facultad de Ciencias, Universidad de Granada, 18002 Granada, Spain 3Department of Geological Sciences, Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854, USA ABSTRACT 2009). However, the relation between foramini- Oceanic anoxic event 2 (OAE2), which occurred during the Cenomanian-Turonian (C-T) fer extinction and water-column anoxia and/or transition and lasted 1 m.y., is characterized by a positive global carbon isotopic excursion and euxinia has not been addressed from an organic stepwise extinctions in marine biota. To examine temporal variations in the dissolved oxygen geochemical point of view. content of the water column, shallow-marine C-T sediments from northern Spain were ana- This study addresses variations in water- lyzed for concentrations of dibenzothiophenes, which are indicators of euxinic depositional column euxinia during the C-T transition in environments, and 2,3,6-trimethylarylisoprenoids, which probably indicate photic-zone eux- sediment from what was then the middle-neritic inia. The positive excursion in δ13C values of carbonates is accompanied by short-term (103– eastern North Atlantic Ocean, using organic 104 yr) and long-term (105 yr) increases in dibenzothiophene and 2,3,6-trimethylarylisopren- geochemical methods. The timing of short- oid concentrations, suggesting that the bottom water and photic zone of the eastern marginal term euxinic events relative to the extinction sea of the North Atlantic Ocean were euxinic. Two of the short-term increases in organic com- of rotaliporid planktonic foraminifers is also pound concentrations took place just after the last occurrence of the planktonic foraminifers investigated. The study section is located east of Rotalipora greenhornensis and R. cushmani. These transient maxima indicate that the extinc- Oviedo, at Arobes, in the province of Asturias, tion of both planktonic foraminifers was due to short-term OAEs lasting 103–104 yr. northern Spain (Fig. 1). The sedimentary suc- cession consists of limestone, marl, and mud- INTRODUCTION minifera showed that the extinction horizon was stone (Fig. 2). Analytical methods are detailed The mid-Cretaceous was generally a time of consistent with low oxygen conditions, and the in the GSA Data Repository.1 extraordinarily warm climates, high sea levels, main cause of this extinction was suggested to and high atmospheric pCO2 levels (e.g., Sch- be the formation of dysoxic intermediate water RESULTS AND DISCUSSION langer et al., 1987; Haq et al., 1987; Clarke and (Kaiho and Hasegawa, 1994). Jenkyns, 1999; Huber et al., 2002; Wilson and Recent organic geochemical studies have Water Depth Norris, 2001). Certain intervals during the mid- sought to provide evidence for anoxic and/ Benthic foraminiferal assemblages in the Cretaceous were characterized by extensive or euxinic conditions in the water column and Arobes section are characterized by Rosalina, deposition of organic-carbon–rich black shale at the seafl oor during OAE2, especially in the Fursenkoina, Praebulimina, Textularia, Hap- across a wide range of marine settings. Because area of what was then the North Atlantic Ocean. lophragmoides, and Lenticulina (see the Data deposition of black shale is favored in oxygen- Molecular fossils (sedimentary derivatives of Repository). This assemblage implies a shal- limited to oxygen-free (anoxic) environments, characteristic biomolecules) of specifi c pig- lower depositional environment than that of ben- these intervals have been interpreted as oceanic ments (isorenieratene) biosynthesized by green thic foraminiferal assemblages (Praebulimina, anoxic events (OAEs; Schlanger and Jenkyns, sulfur bacteria were detected in C-T samples 1976). The most prominent and widespread of from Deep Sea Drilling Project (DSDP) cores the Cretaceous OAEs spans the Cenomanian- from various sites in the North Atlantic Ocean N 0 100 km Turonian (C-T; ca. 93.5 Ma) boundary and is (Sinninghe Damsté and Köster, 1998; Kuypers 4°E called OAE2, or the C-T boundary event. The et al., 2002, 2004) and exploration well cores Bay of Biscay beginning of OAE2 is marked by an increase in in northwestern Africa (Sinninghe Damsté and 44°N SPAIN the stable carbon isotopic ratio of marine sedi- Köster, 1998; Kuypers et al., 2002; Kolonic ment, in both carbonate and organic matter (e.g., et al., 2005). Given that green sulfur bacteria Gijon` Santander 2° Arthur et al., 1988; Kuypers et al., 1999, 2002; require both light and hydrogen sulfi de (H2S) Oviedo Scopelliti et al., 2008). for inorganic carbon fi xation, and are therefore Arobes Bilbao OAE2 induced major changes in the marine indicators of photic-zone euxinia, these studies Vitoria Leon` Pamplona biota, as indicated by an estimated global concluded that photic-zone euxinia developed Logroño extinction rate across the C-T boundary of 26% in the North Atlantic during the C-T transi- 42° 6° Burgos at the generic level and 33%–53% at the spe- tion. Sediment cores through the C-T interval cies level (Sepkoski, 1989, 1996). A study of in northwestern Africa exhibit a high S/C ratio Figure 1. Location of Arobes section in benthic and planktonic foraminifera across the (>0.36) and an abundance of organic sulfur com- northern Spain. C-T boundary showed that ~50% of deep-water pounds, refl ecting deposition in anoxic deposi- foraminifera, but <20% of surface-dwelling tional environments (Nzoussi-Mbassani et al., 1GSA Data Repository item 2011169, analytical foraminifera, became extinct (Kaiho, 1994). 2005). C-T boundary event black shale is also methods and stratigraphic distribution of benthic and planktonic foraminifers, is available online at www The morphology of the calcareous benthic fora- present in South Atlantic and Tethyan settings .geosociety.org/pubs/ft2011.htm, or on request from and has been studied using organic geochemical [email protected] or Documents Secretary, *E-mail: [email protected]. methods (Forster et al., 2008; Sepúlveda et al., GSA, P.O. Box 9140, Boulder, CO 80301, USA. © 2011 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, June June 2011; 2011 v. 39; no. 6; p. 519–522; doi:10.1130/G31805.1; 2 fi gures; Data Repository item 2011169. 519 Downloaded from geology.gsapubs.org on July 21, 2015 Rosalina, Gyroidinoides, Gavelinella, and gradual return to preexcursion values. It is com- to bottom-water anoxia. This interval can be Bolivina) in the Ganuza section, which accu- monly accepted that the positive δ13C excursion presumed to have undergone the most anoxic mulated in a middle to outer shelf environment (phases A and B) refl ects a prominent change conditions of OAE2. (Colin et al., 1982; Lamolda and Peryt, 1995). in the global atmospheric-oceanic pool of inor- The total organic carbon (TOC) content of Together with the abundance of brachiopods, ganic carbon resulting from a global increase in the sediment ranges between 0.11% and 0.53% the foraminiferal assemblage indicates that the the burial rate of 13C-depleted organic carbon (Fig. DR1; see the Data Repository). The TOC Arobes C-T section was deposited on a middle- (Arthur et al., 1988). According to Kuypers et content of mudstone is higher than that of lime- neritic shelf in the eastern North Atlantic Ocean. al. (2002), OAE2 is the main phase of enhanced stone. The vertical profi le of TOC content indi- carbon burial and should be coeval with phases cates that TOC in beds 1, 2, and 16 is relatively Stable Carbon Isotope and Total Organic A and B of the carbon isotope excursion (a span high (>0.4%). No difference is evident between Carbon Content of ~0.7 m.y., as estimated by the occurrence of the TOC contents of phases A, B, and C. The δ13C values of bulk carbonates show planktonic foraminifers and calcareous nanno- a pronounced positive excursion between the plankton; Fig. 2). Organic Molecules base and top of the measured section at Arobes The depth profi le of δ13C values of brachio- Dibenzothiophene (DBT) and alkyl DBTs (Fig. 2A). This excursion has been suggested pod shells from the Arobes section is similar (C1- and C2-) and 2,3,6-trimethylarylisopren- to be global and divisible into three principal to that of carbonates (Fig. 2A); the detailed oids, ranging from C14 to C21 with the excep- δ13 phases (Kuypers et al., 2002): phase A, a rapid brachiopod C profi le of the interval between tion of C17, are present in all sediment samples increase in δ13C values in the uppermost Rot- beds 15A and 19 (top) cannot be determined due from the Arobes section (Figs. 2B and 2C; alipora cushmani biozone; phase B, a plateau to the absence of brachiopods. The absence of Fig. DR2). The vertical profi les of DBTs and of maximum δ13C values in the lower Whit- brachiopods during this interval suggests that the arylisoprenoids are shown in Figures 2B einella archaeocretacea biozone; and phase C, a they were unable to survive in this area owing and 2C, respectively. The vertical trend of A BC D Figure 2. Schematic stratigraphic column for δ13C carbonate ( ) Total C0-C2 Total 2,3,6-trimethyl- spp. spp. Cenomanian-Turonian in- δ13 C brachiopoda ( ) dibenzothiophenes arylisoprenoids terval of Arobes section. (‰ versus PDB) ( : μg/g TOC) ( : μg/g TOC) A: Profi le of δ13C values Age Biozone Lithology Bed number Samples for foraminifera Thickness (m) 3 4 56 0.1 1.0 10 0.01 0.1 1.0 (versus Peedee belem- 34 Praeglobotruncana spp. Whiteinella Dicarinella Rotalipora nite standard, PDB) for carbonate sediment and 15 33 brachiopod shells.