Geochemistry Lithology, microfacies (M1 to M6) and geochemistry data of the Tlacolula Section Scale (m) shale mud wacke packe chert tuff Upper Tamaulipas Formation Agua Nueva Formation Lithostratigraphy Upper Cenomanian–Turonian 80 70 60 50 40 30 20 10 0 B1 89.75 ± 0.64 Ma Filament Bioevents Radiolaria Heterohelicid Framboidal pyrite M1 M2 M3 M4 M5 M6 Microfacies 13 δ C carb (‰ VPDB) -40 -20 0.0 +20 +40 34 δ S (‰ VCDT) 0 0.5 1 1.5 2 2.5 Th (%) 0 10 20 30 U (%) 0 4 8 1216 K (%) 01234 Fe (%) 0 0.8 1.6 Zn (ppm) Mo (ppm) 0 4 8 12 0 400 1200 800 Prevailing anoxic OAE 2 Time- equivalent interval? 39.6 ‰ -25.5 ‰ 42.1 ‰ 37 ‰ 55.8 ‰ 30.6 ‰ -4.7 ‰ 45.2 ‰ 49 ‰ Bioturbation Index (Taylor and Goldring, 1993) Increase 1 5 U/Th Dysoxic Suboxic 0 8 16 24 32 40 Oxic Redox fields proposed by Jones and Manning (1994) s and TOC (%) P (%) V (ppm) 0 0.1 0.2 0 400 800 OA equiva Redox Prevailing anoxic A B C +60 0 1 2 3 4 C/T marine sulfate (Paytan et al., 2004) Sulfur fractionation (ΔS ) sulfate-py Prevailing anoxic INTRODUCTION The Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2) is an exceptional episode of accelerated global change that produced profound variations in the biogeochemical cycles and evolutionary patterns of several organisms (Föllmi 2012, Kedzierski et al., 2012). In central Mexico (Huayacocotla Basin), the Upper Tamaulipas and Agua Nueva formations contains the Cenomanian–Turonian transition and are mainly constituted by black to dark-gray laminated limestone with interbedded thin layers of shale and bentonite. These units also contain organic matter-rich sediments and pyritic layers. In this study, an integrated approach combining sedimentological, petrographic and geochemical analyses provides information about the paleoenvironmental conditions and their relation to the OAE 2. OBJECTIVE To determine the paleoenvironmental conditions that caused the deposition of organic matter and their relation to the global turnover during OAE 2. Gulf of Mexico Pacific Ocean Western Interior Seaway Tlacolula section Paleogeography and paleotectonics of the Late Cretaceus (Turonian) (Blakey, 2016). N Mexico City Poza Rica G. of México Mexico Huayacocotla Basin N 100 km Pacific Ocean Tepexic Santiago Lower Tamaulipas Huayacocotla Cahuasas Huizachal Huehuetepec Pimienta Taman Basement Upper Tamaulipas Otates Agua Nueva San Felipe Mendez Tlacolula Section (83 m) Map of central-eastern Mexico with the actual position of the studied outcrop. Location of the studied section Formation MATERIAL AND METHODS -Field description and sampling of the Tlacolula section (83 m, 139 samples). -Microfacies Analysis (139 thin sections; Olympus BX-60 microscope-UNAM). -Carbonate carbon-isotope data (90 powdered samples extracted from the micrite matrix; Thermo FinniganMAT 253 mass spectrometer-UNIL and UNAM). -Pyrite sulfur isotope composition (12 pyrite samples, Delta C Finnigan MAT continuous flow mass spectrometer-UPC). -Concentrations of major and trace elements (117 powdered samples, portable analyzer ED-XRF Niton XL3T and Niton FXL 950-UNAM). -U–Pb geochronology (1 bentonite sample-33 zircon crystals, LA–ICP–MS spectrometer-UNAM). References ―Blakey, 2016. Paleogeography and Geologic Evolution of North America: www. jan. ucc. nau. edu/rcb7/nam. html. Accessed December, 13, 2016. ―Canfield, 2001. Isotope fractionation by natural populations of sulfate-reducing bacteria. Geochim. Cosmochim. vol. 65, no 7, p. 1117–1124. ―Duque-Botero, et al., 2009. Microspheroids accumulation and geochemistry of an anoxic basin of the Cenomanian/Turonian: The record of the Indidura Formation, NE Mexico. SEPM-Geologic Problem Solving with Microfossils: A Volume in Honor of Garry D. Jones: Tulsa, Society of Sedimentary Geology Special Publication, 93, 171–186. ―Elrick et al., 2009. C-isotope stratigraphy and paleoenvironmental changes across OAE2 (mid-Cretaceous) from shallow- water platform carbonates of southern Mexico. EPSL, 277(3-4), 295–306. ―Föllmi, 2012. Early Cretaceous life, climate and anoxia. Cretac. Res. 35, 230–257. ―Hernández-Romano, et al., 1997. Guerrero-Morelos Platform drowning at the Cenomanian–Turonian boundary, Huitziltepec area, Guerrero State, Southern Mexico. Cretac. Res. 18, 661–686. ―Jones, B., and Manning, D. A., 1994. Comparison of geochemical indices used for the interpretation of palaeoredox conditions in ancient mudstones. Chem. Geol. 111(1–4), 111–129. ―Kedzierski et al., 2012. Bio-events, foraminiferal and nannofossil biostratigraphy of the Cenomanian/Turonian boundary interval in the Subsilesian Nappe, Rybie section, Polish Carpathians. Cretac. Res., 35, 181–198. ―Ohmoto, H., Kaiser, C.J., Geer, A., 1990. Systematics of sulphur isotopes in recent marine sediments and ancient sediment- hosted base metal deposits. In: Herbert, H.K., Ho, S.E. (Eds.), Stable Isotopes and Fluid Processes in Mineralization. University of Western Australia Publ 23, pp. 70–120. ―Paytan et al., 2004. Seawater sulfur isotope fluctuations in the Cretaceous. science, 304(5677), 1663-1665., 304(5677), 1663–1665. ―Taylor, A.M., Goldring, R., 1993. Description and analysis of bioturbation and ichnofabric. J. Geol. Soc. Lond. 150, 141–148. Acknowledgments This study was partially supported by the Generalitat de Catalunya (Research grant 2017SGR0707. Recursos Minerals: Jaciments, Aplicacions, Sostenibilitat). This poster is part of the Master’s thesis of ACR who gratefully acknowledges a fellowship provided by the Consejo Nacional de Ciencia y Técnología (CONACyT)-Secretaría de Energía e Hidrocarburos (SENER) Mexico. We are grateful to Francisco Martín Romero, Gerardo Martínez Jardines and Astrid Vázquez Salgado for help with the P-FRX analyses of the samples. Furthermore, the author the authors express their gratitude to: Mario Martínez-Yañez and Karina Navarrete Flores for field assistance; Mario A. Ramos Arias and Michelangelo Martini for help in U-Pb data processing; Margarita Reyes Salas and Sonia Angeles García for their assistance with SEM; Francisco Otero Trujano and Edith Cienfuegos Alvarado for carbon isotope analysis, and Lourdes Omaña Pulido for preliminary biostratratigraphic data. Agua Nueva Formation Upper Tamaulipas Formation Field (A) Upper Tamaulipas Formation: Brown limestone with chert nodules, ocassionally intercalated with argillaceous limestone and layers of black chert. Agua Nueva Formation: Black, laminated limestone bearing chert nodules intercalated with: (1) thin calcareous shale, (2) greenish bentonite horizons, and (3) dark, laminated beds rich in organic matter and pyrite, alternating with bioturbated beds. (B) Chert nodule (C) Bedding–parallel burrow filled with pyrite (D) Burrows on top of the limestone bed (E) Interbedded limestone and bentonite beds A B C D E RESULTS U–Pb geochronology and preliminar biostratigraphic data: (A) Cathodoluminescence images of some zircons, (B) Concordance diagram for the analyzed zircons, (C) Mean age diagram. (D) Helvetoglobotruncana, (E) Marginotruncana. Latest Cenomanian–Turonian Age 0.08 0.09 0.10 0.11 0.12 0.13 0.14 0.014 0.016 0.018 0.020 l 80 l 90 l 100 l 110 l 120 l 130 l l l l l l l l l l l l l 207 235 Pb/ U SAMPLE B1 (at m) 36 206 238 Pb/ U Mean = 89.75 ± 0.64 | 1.26 Ma (n= 12 ) MSWD = 2.10 N 1 2 3 4 5 6 7 8 9 10 11 12 85 90 95 Age A B C D E 500 µm 500 µm hd hd hd c f bv f bv pf r r r pf r pf f d d r m m h h h M6: Medium gray, highly bioturbated packstone/wackestone with planktonic foraminifera (BI=4-5) b b b Microfacies -Hedbergella (hd), Clavihedbergella (c), planktonic foraminifera (pf), filaments (f), bivalve shell fragment (bv), calcitized radiolarian (r), Marginotruncana (m), Dicarinella (d), Heterohelix (h), and burrows (b). M5: Dark gray, bioturbated wackestone with heterohelicids and radiolarian (BI=2-3) M4: Dark gray, bioturbated, mudstone/wackestone with whitenellids, dicarinellids and radiolarian (BI=2-3) M3: Dark gray packstone/wackestone with calcitized radiolaria and planktonic foraminifera M2: Black, lamimated wackestone with planktonic foraminifera and bivalve ‘filaments’’. M1: Black to dark-gray mudstone/wackestone with hedbergellids and radiolarian. SEM images showing the main features of the framboidal pyrite. Note the octahedral microcrystals 2 µm 2 µm 2 µm 2 µm 2 µm 2 µm Pyrite is present in the form of framboids, occurring as nodules, disseminations, thin horizons or filling burrows. DISCUSSION AND CONCLUSIONS The Tlacolula section was constrained by U-Pb zircon geochronology (89.75±0.64 Ma) and preliminary biostratigraphic data to the latest Cenomanian–Turonian. It consists mainly of pelagic mudstone to packestone containing radiolarians, planktonic foraminifera and bivalve filaments, with chert nodules, intercalated with thin calcareous black shale and greenish bentonite horizons. In general, this section was deposited in reducing environments, as indicated by the trace-metal parameters (Fe, P, V, U, Mo and Zn), the U/Th ratio (mostly above 0.75), the TOC content (up to 3.96 %) and the pyrite framboid size mostly around 2 µm. At least three intervals can be associated with strongly oxygen-depleted conditions (A, B and C). Indeed, they match with the regular presence of microfacies M2 and M3 (rich in bivalve filaments and radiolaria and poorly bioturbated, BI=1-2) in the lower and middle part of the section suggest also suggesting reduced conditions. However, the increase in bioturbation (microfacies M5 and M6; BI=3-5) in the upper part indicates less oxygen deficiency. Due to its lowermost stratigraphic position, the interval A (lowest 25 m) can be correlated with the OAE 2. It can be associated with the end of the positive carbon isotope excursion (CIE) that characterizes this event. Remarkably, the interval A coincides with a short-time CIE (1.3‰), likely reflecting higher eutrophic conditions. Oxygen deficiency during deposition on intervals B and C was controlled mainly by local redox conditions inherent to central Mexico. Similar conditions during the early Turonian have been also reported for southern (Mexcala Formation- Guerrero-Morelos Platform; Hernández-Romano et al., 1997; Elrick et al., 2009) and northern Mexico (Indidura Formation-Parras Basin; Duque-Botero et al., 2009). Therefore, a regional control of redox conditions during the early Turonian is suggested for this part of the proto-North Atlantic Basin, possibly associated with the intermittent permanence of a weaker oxygen-minimum zone. 34 On the other hand, the S-depleted values found in the pyrite indicate formation mediated by microbial sulfate reduction (MSR) in an open system with available sulfate, favored by a high organic matter burial and the oxygen-depleted conditions. The calculated sulfate-pyrite fractionation match those found in laboratories by MSR of 4 to 46‰ (Ohmoto et al., 1990; Canfield, 2001). PALEOREDOX CONDITIONS DURING THE CENOMANIAN‒TURONIAN IN CENTRAL MEXICO AND THEIR RELATION TO OCEANIC ANOXIC EVENT 2 a b c d Colín-Rodríguez, A. , Núñez-Useche, F. , Adatte, T. , Pura, A. a b c d Posgrado en Ciencias de la Tierra-Universidad Nacional Autónoma de México (UNAM), Instituto de Geología-UNAM, Institute of Earth Sciences-Université de Lausanne, Department of Mining Engineering and Natural Resources-Universitat Politècnica de Catalunya Email: colinrdgza@gmail.com, fernandonu@geologia.unam.mx