Stable Oxygen Isotope Chemostratigraphy and Paleotemperature Regime of Mosasaurs at Bentiaba, Angola

Netherlands Journal of Geosciences —– Geologie en Mijnbouw | 94 – 1 | 137-143 | 2015 doi: 10.1017/njg.2015.1 Stable oxygen isotope chemostratigraphy and paleotemperature regime of mosasaurs at Bentiaba, Angola C. Strganac1,2,*,L.L.Jacobs1,M.J.Polcyn1,K.M.Ferguson1,O.Mateus3,4,A.Ol´ımpio Gonçalves5, M.-L. Morais5 & T. da Silva Tavares5,6 1 Roy M. Huffington Department of Earth Sciences, Southern Methodist University, Dallas, Texas 75275, USA 2 Perot Museum of Nature and Science, Dallas, Texas 75201, USA 3 GeoBioTec, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Caparica, Portugal 4 Museu da Lourinh~a, Rua Jo~ao Luis de Moura, 2530-157, Lourinh~a, Portugal 5 Departamento de Geologia, Faculdade de Ciencas, Universidade Agostinho Neto, Avenida 4 de Fevereiro 7, Luanda, Angola 6Université de Bourgogne, Dijon, France * Corresponding author. Email: [email protected] Manuscriptreceived:2June2014,accepted:6January2015 Abstract Stable oxygen isotope values of inoceramid marine bivalve shells recovered from Bentiaba, Angola, are utilised as a proxy for paleotemperatures during the Late Cretaceous development of the African margin of the South Atlantic Ocean. The d18O values derived from inoceramids show a long-term increase from –3.2‰ in the Late Turonian to values between –0.8 and –1.8‰ in the Late Campanian. Assuming a constant oceanic d18Ovalue,an∼2‰ increase may reflect cooling of the shallow marine environment at Bentiaba by approximately 10°. Bentiaba values are offset by about +1‰ from published records for bathyal Inoceramus at Walvis Ridge. This offset in d18O values suggests a temperature difference of ∼5° between coastal and deeper water offshore Angola. Cooler temperatures implied by the d18O curve at Bentiaba coincide with the stratigraphic distribution of diverse marine amniotes, including mosasaurs, at Bentiaba. Keywords: Cretaceous, South Atlantic Ocean, oxygen, paleotemperature, inoceramid Introduction vertebrates from the eastern South Atlantic (Jacobs et al., 2006). The oldest mosasaurs knownfromtheSouthAtlantic We present a temporally calibrated, shallow marine d18Ochemo- Ocean, Angolasaurus bocagei and Tylosaurus iembeensis,are stratigraphic curve for Bentiaba, southern Angola (Fig. 1), found at Iembe, north of the capital Luanda, in Late Turonian relating stable oxygen isotopes of this portion of the southern nearshore deposits that also produced the sauropod dinosaur hemisphere to values from the Walvis Ridge offshore Angola and Angolatitan adamastor (Mateus et al., 2011). Marine amniote elsewhere around the globe. The presented d18O values are used remains known from Iembe also include halisaurine vertebrae to estimate the paleotemperature regime in which mosasaurs and a marine turtle, Angolachely mbaxi (Mateus et al., 2009, and other marine amniotes along this portion of southwest 2012; Polcyn et al., 2012). Africa lived throughout much of the Late Cretaceous. The greatest concentration of mosasaurs in Angola comes Mosasaurs have a fossil record extending from the Cenoma- from Upper Campanian–Maastrichtian sediments at Bentiaba nian, approximately 98 Ma, to their extinction at the end of (Strganac et al., 2014b), but there are significant exposures the Cretaceous at 66 Ma (Jacobs et al., 2005a,b; Polcyn et al., below the fossil-bearing horizons, which range in age from Cen- 1999, 2014). Since 2005 our team, Projecto PaleoAngola, has omanian to Santonian. The chronological context for Bentiaba conducted field expeditions in Cretaceous and younger rocks is provided by the magnetostratigraphy and a d13C chemostrati- of coastal Angola, greatly improving the fossil record of graphic framework anchored by an 84.5 Ma 40Ar/39Ar whole-rock © Netherlands Journal of Geosciences Foundation 2015 137 Downloaded from https://www.cambridge.org/core. IP address: 170.106.34.90, on 03 Oct 2021 at 06:58:28, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/njg.2015.1 Netherlands Journal of Geosciences —– Geologie en Mijnbouw 94 – 1 | 2015 latitudinal temperature gradients. The current study provides temperature estimates of shallow marine environments, below wave base, for much of Late Cretaceous time as the South Atlan- tic was growing and was inhabited by diverse assemblages of marine amniotes. Cretaceous d18O records have been used to elucidate changes in ocean circulation as well as temperature (Cramer et al., 2009; Friedrich et al., 2009, 2012). Most paleotemperature records utilise benthic foraminifera, whose d18O values reflect offshore deep marine environments with little to no influence by meteoric input or fluctuations in salinity (Gross- man, 2012). These methods can be extended to shallow shelf setting unaffected by high freshwater influx (Sessa et al., 2012), which we assume to be the case at Bentiaba (Strganac et al., 2014b). The d18O curve from Bentiaba provides a shallow marine temperature curve that can be compared to the long- term benthic records to track broader temperature trends, to Fig. 1. Location map of Bentiaba, Angola. Inset shows location of Angola obtain indications of larger circulation patterns, and to pro- within Africa, offshore basins labelled in capital letters. Note location of vide a paleotemperature context for evolution in the marine DSDP Hole 530A, which is discussed in the text. Modified from Strganac realm. et al. (2014a). The Late Cretaceous marks the transition from global high temperatures during the Late Cenomanian-Turonian climatic radiometric date on an intercalated basalt of the Ombe Formation, maximum and progressing through the cooler temperatures which is consistent with ammonite biostratigraphy (Strganac that characterise much of the Cenozoic. Cretaceous tempera- et al., 2014a). The Bentiaba d18O curve presented here was con- ture trends coincide with significant paleoceanographic structed as a complement to the d13C record presented by Strganac events, including Oceanic Anoxic Event 2 at the Cenomanian- et al. (2014a). Turonian Boundary, which is a widespread period of rapid The mosasaur fauna of the Campanian Baba Formation is burial of organic matter related to marine productivity and en- fragmentary and includes both russellosaurians and mosasaurines, hanced ocean stratification (Arthur et al., 1988; Erbacher while that of the overlying Maastrichtian Mocuio Formation is pre- et al., 2001, Forster et al., 2008; Friedrich et al., 2012; Strga- dominately mosasaurine (Strganac et al., 2014b). Within the Mocuio nac et al., 2014a). Connections between major ocean basins in- Formation, the mid-Maastrichtian (approximately 71.5 Ma) Bench creased after the Turonian as a result of tectonic drift of 19 fauna is particularly rich (Schulp et al., 2006, 2008, 2013; Polcyn continents, allowing cool waters derived from high latitudes et al., 2010, 2014; Mateus et al., 2012), and it is the subject of to circulate equatorially, contributing to declining global tem- a paleoecological analysis (Strganac et al., 2014b). The paleogeo- perature (Hay et al., 1999; Miller et al., 1999; Cramer et al., graphic context of Angolan vertebrates was initially examined in 2009; Robinson & Vance, 2012). The d18Ostratigraphiccurve Jacobs et al. (2009, 2011) and Polcyn et al. (2010), and their paleo- presented here is a proxy for shallow marine temperature ecology was examined initially in Robbins et al. (2008) and Polcyn change at Bentiaba during the Late Cretaceous development et al. (2010, 2014). of the South Atlantic. Little paleotemperature research relevant to this study has The mosasaur-bearing portion of the Bentiaba section lies been conducted in Africa. An analysis of paleosols in the Samba above the Ombe basalt in the Baba and Mocuio formations drill core from the Congo Basin (Cahen et al., 1959), represent- (Fig. 2). Carbonate cement creates benches in the Mocuio ing the interior of Gondwana prior to the opening of the South Formation, which are numbered in the section. Vertebrates Atlantic, indicates Jurassic average annual soil temperatures occur throughout the Baba and Mocuio formations. The Cam- between 25 and 4063°C, with an average of 33°C, s =7.6°C panian Baba Formation preserves a fragmentary but mixed rus- (Myers et al., 2011), accompanied by low biological productivity sellosaurine-mosasaurine fauna extending into the lower part (Myers et al., 2012). Myers et al. (2011) also presented a Lower of the Mocuio Formation. In the Lower Maastrichtian part of Cretaceous soil temperature estimate of 2363°C. the Mocuio Formation, the Bench 19 Fauna is dominated by Temperature tolerances for mosasaurs of Cenomanian age, mosasaurines. The highest levels of the Mocuio fauna, which near the time of origin for the group, were considered by Jacobs we interpret as Upper Maastrichtian (Strganac et al., 2014a), et al. (2005b). Kolodny & Luz (1991) reported the d18Ovalueof preserve the globidensine Carinodens and a large, derived the Cenomanian shark Cretolamna appendiculata from Angola, Prognathodon (Mateusetal.,2012;Schulpetal.,2013; included by Pucéat et al. (2007) in their study of Cretaceous Strganac et al., 2014b). 138 Downloaded from https://www.cambridge.org/core. IP address: 170.106.34.90, on 03 Oct 2021 at 06:58:28, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/njg.2015.1 Netherlands Journal of Geosciences —– Geologie en Mijnbouw 94 – 1 | 2015 Fig. 2. Section at Bentiaba with d13Candinoceramidd18O stratigraphic

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