Halite fluid inclusions and the late Aptian sea surface temperatures of the Congo Basin, northern South Atlantic Ocean

Cretaceous Research 71 (2017) 85e95

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Halite fluid inclusions and the late Aptian sea surface temperatures of the Congo Basin, northern South Atlantic Ocean Hua Zhang a, b, *, Fenglin Lü a, c, Steffen Mischke d, Meiling Fan c, Fan Zhang c, Chenglin Liu a a

MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China €t Berlin, Malteserstr. 74-100, 12249 Berlin, Germany Department of Geological Sciences, Freie Universita c China University of Geosciences, Beijing 100083, China d Faculty of Earth Sciences, University of Iceland, Sturlugata 7, Askja, 101 Reykjavík, Iceland b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 June 2016 Received in revised form 14 November 2016 Accepted in revised form 17 November 2016 Available online 17 November 2016

The Aptian stage represents a pivotal transition of the Cretaceous climate of the Earth but a comprehensive picture of the climate conditions is still unavailable due to the fragmentary paleotemperature records of the mid-late Aptian interval, especially in the Central Segment of South Atlantic region In this region extensive and thick salt sequence severely limiting the applicability of traditionally used paleotemperature proxies. In this paper, we present new quantitative sea surface temperatures from Congo Basin, during the late Aptian based on homogenization temperatures (Th) of fluid inclusions of halite. Homogenization temperatures concentrated at 25e35
C are consistent with previous climate model results and represent the prevailing sea surface temperatures in the Congo Basin. The derived maximum sea surface temperature is 46.5
C and much higher than existing paleotemperature records for the late Aptian, apparently representing local hot climate conditions. However, it is comparable with temperature records from many ancient and modern evaporitic basins where potash deposits also largely developed as seen in the Congo Basin. Significant cooling by about 5
C was identified by both maximum (ThMax) and average (ThAverage) homogenization temperatures of halite during the late Aptian. A comparison of our results with records from other places leads to the conclusion that the observed declining sea surface temperatures of the Congo Basin represent the regional response to global climate cooling in the late Aptian. However, the amplitude, short-term oscillation pattern and worldwide correlation of this cooling is still difficult to assess due to often imprecise and poorly resolved age data, emphasizing the need of better constraint chronologies. Homogenization temperatures of halite fluid inclusions demonstrate the potential of the method to reliably track environmental and climatic changes of the past, especially in evaporitic settings of the Phanerozoic where other climate proxies are often not available. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Halite fluid inclusions Sea surface temperatures Quantitative reconstructions Late Aptian Northern South Atlantic Ocean

1. Introduction Understanding ancient analogs of greenhouse climate conditions and knowledge of past climate variability are essential for reliable predictions of future environmental and climate changes, especially on a warming Earth. Beside frequently chosen analogs

* Corresponding author. MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China. E-mail address: [email protected] (H. Zhang). http://dx.doi.org/10.1016/j.cretres.2016.11.008 0195-6671/© 2016 Elsevier Ltd. All rights reserved.

(e.g. the late Paleozoic; Gastaldo et al., 1996; the Paleocene-Eocene Thermal Maximum; Bowen et al., 2006; Meng et al., 2011a), the Cretaceous period represents a classic “greenhouse state” of the Earth and is a relevant model for a return to greenhouse climate conditions (Jenkyns, 2003; Hay, 2011). Over the past decades, research on Cretaceous climate change has made tremendous progress (Leckie et al., 2002; Hu et al., 2005, 2012; Wang et al., 2011; Chaboureau et al., 2013) and revealed multi-phased climate change despite long-term warm climate (Pearson et al., 2001; Wilson and Norris, 2001; Bice, 2002; Huber et al., 2002; Norris et al., 2002; Wilson et al., 2002; Skelton et al., 2003; Bice et al., € llmi, 2012; Friedrich et al., 2012; Wang, 2013). 2006; Hay, 2008; Fo

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In particular, the Aptian was a key period of the Earth system when the land-sea was reconfigured due to the breakup of the Gondwana supercontinent (Nürnberg and Müller, 1991; Chaboureau et al., 2012, 2013). The potential effects of this on past climate change have stimulated decades of research. Paleoclimatic reconstructions of the Aptian Stage based on various temperature proxies have been conducted in the Tethys region (Savin, 1977; Menegatti et al., 1998; Ferguson et al., 1999; Jenkyns, 2003; Herrle et al., 2004; Erba et al., 2010; Bottini and Mutterlose, 2012; Grossman, 2012; Hu et al., 2012; Husinec et al., 2012; Pauly et al., 2013; Godet et al., 2014), the North Sea (Mutterlose and Bottini, 2013), the Southern Sea (Jenkyns et al., 2012), the European Russian Platform (Zakharov et al., 2013), the Pacific (Price, 2003; Dumitrescu et al., 2006; Takashima et al., 2007; Ando et al., 2008; Bottini et al., 2012, 2014) and the Atlantic Ocean (Carvalho et al., 2006; Friedrich et al., 2008; Trabucho Alexandre et al., 2010; Chaboureau et al., 2012, 2013). The studies revealed a complex climatic variability in space and time (Hochuli et al., 1999; Jenkyns et al., 2012; Bottini et al., 2014). Although considerable studies of temperature variations that span the Aptian have been conducted, a comprehensive picture of climatic change during the 12-Ma long Aptian Stage is still unavailable (Malinverno et al., 2012; Bottini et al., 2014), due to the fragmentary paleotemperature records of the late Aptian, especially in the Central Segment of the South Atlantic Ocean. In this region, sedimentary environments with an extremely high salinity represented by extensive and thick salt beds, including potash deposits, severely limit the applicability of traditionally used paleotemperature proxies such as oxygen isotopes, the TEX86 index, calcareous nannofossils and palynomorphs. Salt minerals formed at the Earth's surface can be a treasure chest of paleoclimate and sometimes paleoweather conditions due to their high sensitivity to temperature (Roberts and Spencer, 1995; Benison and Goldstein, 1999). In particular, primary fluid inclusions within salt minerals are remnants of the hydrosphere and atmosphere, and, yield detailed information about water temperature, water chemistry and atmospheric conditions under which they were formed (Roberts and Spencer, 1995; Lowenstein et al., 1998; Benison and Goldstein, 1999). Homogenization temperatures (Th) of halite fluid inclusions serve as indictor of water temperatures and approximate air temperatures without corrections for substantial pressure compared with climate proxy data obtained through other ways and provide direct and quantitative temperature inferences. It is thus widely used for temperature reconstructions of the geological past including PrecambrianeCambrian (Meng et al., 2011a), Silurian (Losey and Benison, 2000; Satterfield, 2005), Permian (Benison and Goldstein, 1999; Zambito and Benison, 2013), Paleogene (Meng et al., 2013; Zhao et al., 2014), Quaternary (Lowenstein et al., 1998, 1999) and modern settings (Roberts and Spencer, 1995). Continuous salt deposits of the upper Aptian in the Congo Basin contain well-preserved primary fluid inclusions which provide a unique opportunity for quantitative climate reconstructions in a setting where other climate proxies are less promising. Therefore, our investigation aims to estimate the late Aptian sea surface temperatures and the climate conditions of the Congo Basin, northern South Atlantic Ocean based on homogenization temperatures of halite fluid inclusions. Our study represent new direct and quantitative paleo-temperature reconstructions and thus, significant paleoclimate data that supplement the knowledge of the Aptian climate conditions and advance our understanding of climate change in the Cretaceous greenhouse world.

2. Geological setting of study area The South Atlantic Ocean results from the breakup of Gondwana into two continents, South America and Africa, during the Cretaceous period (Chaboureau et al., 2013). It can be divided into four latitudinal segments (Moulin et al., 2005) with the central segment characterized by extensive and thick evaporite deposits along both continent margins (Fig. 1). Salt accumulation in the central segment of the South Atlantic Ocean occurred in a single large pre-breakup (syn-rift) basin located in a near subtropical position between 10
and 27
S (Torsvik et al., 2009). The salt deposits of the western continental margin of Africa are confined to the Douala Basin, the Gabon Basin, the Congo Basin, and the Kwanza Basin (de Ruiter, 1979). The Congo Basin is a broad downwarp centered on the Congo craton, extending into the two Congo Republics, the Central African Republic and Angola (Giresse, 2005). It is topographically surrounded by higher areas: the rift flanks of the Central African Rift to the north, the East African Rift to the east, the South African Plateau to the south, and the Mayombe Mountains to the west (Buiter et al., 2012). The Congo Basin was initially formed during the Late Jurassic rifting and subsequent Cretaceous opening of the South Atlantic Ocean (Harris, 2000). Stratigraphically, two major intervals of the Mesozoic are recognized in the Congo Basin, the pre-salt section and the post-salt section (McHargue, 1990). The Aptian Chela and Loeme Formations are between these two major intervals (Fig. 2), marking the major incursion of the South Atlantic Ocean into the widening rift (Harris, 2000). The lower part of the pre-salt sequence is characterized by Hauterivian to Barremian coarse sandstones, shale and turbidite sandstones, which were deposited during a period of active block faulting and localized subsidence (Harris, 2000). The upper part of the pre-salt sequence unconformably overlays the lower part and consists of the Barremian Marnes Noires and Argilles Vertes Formations (Fig. 2). The two formations represent laminated, carbonaceous marl and shales with thin turbidite sandstone beds, which were deposited during a period of regional downwarping with minimal faulting (Harris, 2000). The Chela Formation is generally marked by a transgressive sandstone/ shale sequence, representing the initial marine incursion (Harris, 2000). It unconformably overlays the Argilles Vertes Formation with a sedimentary hiatus of about 7e8 Ma inferred from the correlation of biostratigraphic zonation of basins in proto-South Atlantic (Poropat and Colin, 2012). The Loeme Formation in the Congo Basin is characterized by salt deposits which formed during the latest stage of the breakup of the Gondwana supercontinent in the late Aptian (Davison, 2007). The age of the Leome salt is probably the late Aptian period based on compiled ostracod zone (Poropat and Colin, 2012) and magnetic anomalies (Torsvik et al., 2009). Salt of this formation has a thickness of 1e2 km in the deepest part of the Congo Basin (Brognon and Verrier, 1966; Asmus and Ponte, 1973; Mohriak et al., 2008) and well-preserved evaporitic cycles with each cycle of cm-scale asphalt-rich shale at the bottom and the overlying m-scale halite and carnallite can be correlated between wells in the Congo Basin (de Ruiter, 1979). The mineralogical composition of these salts is characterized by abundant chloride minerals (halite, carnallite), unusual calcium chloride salts such as bischofite and tachyhydrite beds with thickness of over 100 m, and a small amount of gypsum (Wardlaw, 1972; de Ruiter, 1979). The overlying Madiela Formation is mainly characterized by clastic rocks and carbonates, representing increasingly fully marine sequence. The study area, the Cuckoo Marcy mining region, is located in the Kouilou Department in the southwest of the Republic of the Congo. Tectonically, it belongs to the central part of the Congo Basin. No significant fault has been observed in the area. By far, several cores have been drilled for commercial potash

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87

Fig. 1. General structural map of the South Atlantic Ocean (modified from Moulin et al., 2005), and simplified geology of the study area with location of core ZK143.

exploration in studied area and showed that the Leome formation salt was well preserved without deformation caused by later tectonic activities. Paleontological evidence from the Aptian to lowest Albian dolomite and sapropel sequence and the absence of thick marine layers prior to the Aptian Chela layer (the Chela layer found on both margin of South Atlantic is very thin), indicate the late Aptian salt in the Congo Basin and other salt basins of South Atlantic area deposited in a shallow-water environment with no more than 500 m of paleo-water depth (Marton et al., 2000; Moulin et al., 2005). 3. Materials and experimental method 3.1. Core materials and halite samples Several cores were drilled in the Cuckoo Marcy mining region for the commercial potash-deposit exploitation. Core ZK143 was recovered from the southern part of the Cuckoo Marcy mining area at 04
290 S and 11
520 E. To avoid dissolution and reprecipitation, drilling mud with halite-saturated brine was used to recover the investigated core. The cored strata represent the Albian Madiela Formation in the upper part above 227 m and the upper Aptian Loeme Formation below. The Madiela Formation consists of gypsum-bearing clays, mudstones and carbonates. The underlain Loeme Formation is characterized by a ca. 370-m thick salt sequence. According to lithological comparisons with other published cores (de Ruiter, 1979), only the lowermost part of the Loeme Formation salt is lacking in core ZK143. The lithology and cyclic sequence of salts recovered in the core are shown in Fig. 2. In total, three cycles within the salt sequence were recognized from the bottom to the top with each cycle starting with asphalt-rich shale interbedded with thin halite beds, overlain by halite and carnallite. The thickness of carnallite deposits decreases upward from the first to the third cycle. Tachyhydrite occurs as a horizontal layer with a thickness of ca. 7 m at the top of the first cycle. In total, seven halite samples were collected from core ZK143 from stratigraphic levels at 591.11, 579.21, 576.67, 388.97, 287.34, 275.06, 250.06 and 245.20 m to perform homogenization temperature analysis. All sampled halites are characterized by thin beds

with an individual thickness of 10e15 cm. Cumulate crystals and chevron crystals dominate these samples. Samples 1 (591.11 m) and 6 (275.06 m) represent cumulate crystals with random orientation. The other samples contain both cumulate and chevron crystals with cumulate crystals occurring at the base of the chevron crystals. Cumulate crystals are mm-scale and chevron crystals in a scale of cm. Both types are characterized by alternating inclusion-rich and inclusion-poor growth bands, and fluid-inclusion rich cores (Fig. 3). Inclusion-rich bands often have a cloudy appearance with white to milk-white color, while inclusion-poor bands are often clear and transparent. Primary fluid inclusions in these crystals are usually 5e35 mm in diameter and dominated by single-phase liquid inclusions. Two- or three-phase inclusions and liquid-vapor inclusions were rarely observed at laboratory temperature. In addition, transparent halite with mosaic textures was also observed in the core ZK143 but was not selected in this paper. This type of halite is often regarded to have formed diagenetically as a result of the filling of dissolution cavities by halite cements and displacement growth of new halite crystals (Casas and Lowenstein, 1989; Lowenstein et al., 1998), thus, not necessarily providing information about the original water body where halite was formed. Therefore, only the primary single-phase aqueous liquid inclusions within cloudy fluid-inclusion bands in chevron and cumulate crystals were selected in our study for homogenization temperature analysis. 3.2. Experimental methods The ‘cooling nucleation’ method utilized in our study follows Lowenstein et al. (1998) and Benison and Goldstein (1999). Careful attention was paid to avoid dissolution and overheating during sample preparation. Unpolished halite samples were separated into ca. 0.5e1 mm thick fragments with a hammer chiseling along the cleavage planes. Before cooling stage, detailed petrographic studies were conducted for each halite fragment sample to document individual primary liquid fluid inclusions, including mapping and photographically recording the size, shape and distribution of fluid inclusions and preexisting bubbles within inclusions. All samples were then placed in an airtight plastic box with desiccant for moisture protection. Samples were

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4. Results 4.1. Homogenization temperature data Approximately 10e15% of primary single-phase liquid fluid inclusions produced artificially nucleated vapor bubbles during cooling. In total, 369 Th data were obtained from nucleated vapor bubbles in the first run (Table 1 and Appendix). The Th of the seven stratigraphic intervals have a range of 19.0e41.9
C (245.20 m), 21.3e42.0
C (275.06 m), 20.6e44.3
C (287.34 m), 23.6e45.9
C (388.97 m), 22.3e46.5
C (576.67 m), 19.0e39.6
C (579.21 m) and 21.8e33.9
C (591.11 m), respectively (Fig. 5). The corresponding mean Th values are 29.9
C, 26.6
C, 26.4
C, 28.2
C, 31.0
C, 31.6
C and 29.5
C, respectively. Within the same sample, Th ranges of individual chips range within 15
C or less, and with a minimum value of 4.0
C and maximum value of 23.7
C. Each inclusion-rich band is considered here an individual fluid inclusion assemblage (FIA; Goldstein and Reyonolds, 1994), suggesting all fluid inclusions along it formed at about the same time (Benison and Goldstein, 1999). All Th within individual fluid inclusion assemblages were within 15
C, and 92% of fluid inclusion assemblages have ranges of less than 10
C. The majority of fluid inclusions from assemblages close to each other yielded similar Th data. When chevron crystals encounter cumulate crystals in the same bed, Th values share a similar range. For example, Th range from 21.2 to 39.6
C and 23.4e37.1
C for fluid inclusions from chevron and cumulate crystals in sample 2 (579.21 m), respectively. 4.2. Reproducibility Following a second run of ‘cooling nucleation’, 19 out of 44 fluid inclusions failed to nucleate vapor bubbles and 25 Th data were obtained (Table 2). Th data obtained from the second cooling are lower than those of the first run. The differences of Th data for two runs are <2
C for 22 fluid inclusions and 3e4
C for the other 3 fluid inclusions. 5. Discussion 5.1. Representativity and validity of the Th data

Fig. 2. Stratigraphic position of the cored Loeme Formation and locations of samples from core ZK143.

placed in a Haier freezer at 20
C for 2e4 weeks to prevent the alteration of the inclusion volume or shape and the formation of artificially nucleated vapor bubbles (Fig. 4). After removal from the freezer, the samples were quickly placed in a Linkam ThMSG600 device for subsequent heating and freezing stages. All samples were rapidly cooled to 20
C and held at this temperature until the nucleated vapor bubbles within the inclusions were located and observed. Then, the heating stage was achieved by warming at a rate of 0.5
C/min up to a temperature of 15
C. Thereafter, the rate was lowered to 0.1
C/min until all artificially nucleated vapor bubbles disappeared (homogenized). The Linkam ThMSG600 stage in this study was calibrated precise to approximately ±1.0
C between 0 and 50
C. The reproducibility of the Th data was examined for sample 2 (579.21 m) which was returned to the freezer to re-nucleate vapor bubbles after their homogenization following the first run.

Alterations of fluid inclusions as a result of its high solubility and its potential for deformation can cause difficulties in interpreting homogenization temperatures of halite fluid inclusions as ancient brine temperatures or approximate air temperatures. Therefore, homogenization temperatures of primary fluid inclusions in ancient halite serve as reliable temperature proxy only if alterations of fluid inclusions either during burial or sample preparation can be ruled out. Since the thick section preparation has no effect on the homogenization temperatures of fluid inclusions (Benison and Goldstein, 1999), uncut, unpolished halite cleavage chips were used for Th analysis to avoid overheating or dissolution that may occur during traditional grinding and polishing of thin sections on grit or diamond paper with oil. All selected fluid inclusions for Th analysis are single-phase liquid inclusions and from the wellpreserved inclusion-rich bands or cores of cumulate or chevron crystals, because primary fabrics are often defined by concentrations of fluid inclusions along growth planes in the salt (Roberts and Spencer, 1995). Gas inside fluid inclusions often leads to anomalously high Th (Roedder and Belkin, 1979; Lowenstein et al., 1998). After cooling, photos of fluid inclusions taken prior and posterior to cooling were compared to ensure that no fluid inclusions underwent alteration or freezing during cooling in freezer. In addition, the thermal re-equilibration is generally regarded as another

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89

Fig. 3. Primary fluid inclusion in the Loeme Formation (A, Chevron crystal, boxed arrow indicates stratigraphic up; B, Cumulate crystal; C, Inclusions-rich core of cumulate crystal).

Fig. 4. Changes of primary fluid inclusion during the homogenization process (A, Vapor bubbles formed after ‘cooling nucleation’; B, Vapor bubbles (arrows) disappeared after heating).

Table 1 Summarized homogenization temperature of halite fluid inclusions of the Leome Formation in the Congo Basin. Sample

Depth/m

Number

ThMax/
C

ThMin/
C

ThAve/
C

ThRange/
C

1 2 3 4 5 6 7

591.11 579.21 576.67 388.97 287.34 275.06 245.20

65 44 57 38 64 42 59

33.9 39.6 46.5 45.9 44.3 42.0 41.9

21.8 19.0 22.3 23.6 20.6 21.3 19.0

29.5 31.6 31.0 28.2 26.4 26.6 29.9

12.1 20.6 24.2 22.3 23.7 20.7 22.9

ThMax ¼ Maximum homogenization temperature; ThMin ¼ Minimum homogenization temperature; ThAve ¼ Average homogenization temperature; ThRange ¼ Homogenization temperature range.

potential source that may cause alteration of fluid inclusions. This possibility was evaluated based on the consistency of Th of fluid inclusion assemblages. Our data show that all Th within individual fluid inclusion assemblages were within 15
C, and 92% of fluid inclusion assemblages have Th ranges <10
C. The results thus fall

well within the guidelines of Goldstein and Reynolds (1994) who suggested that homogenization temperatures are likely not altered by thermal re-equilibration if 90% of the Th data within single fluid inclusion assemblages vary within a 10e15
C interval. In addition, fluid inclusions of halite would be internally overpressured relative to ambient conditions due to overheating during sample preparation and burial. This results in stretching of fluid inclusions to relieve the high internal pressures (Bodnar and Bethke, 1984; Goldstein, 1986; Mclimans, 1987). The increased volume of fluid inclusions to decrease the internal overpressure produces a lower density that would yield a higher homogenization temperature (Goldstein, 2001). Since smaller inclusions are more resistant to thermal reequilibration, stretching of the larger halite fluid inclusions more likely results in higher homogenization temperatures for the larger inclusions compared with those for the smaller ones (Petrichenko, 1979; Roedder and Belkin, 1980; Roedder, 1984; Benison and Goldstein, 1999). An evaluation of the relationship between inclusion size and homogenization temperature is therefore required to examine the possible alteration

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Fig. 5. Histogram of homogenization temperatures plotted against number of fluid inclusions.

Table 2 Summarized homogenization temperature of sample 2 for the two runs. Sample 2 (579.21 m)

Th/Homogenization temperatures (
C)

FIA 1 FIA 1* FIA 2 FIA 2* FIA 3 FIA 3* FIA 4 FIA 4* FIA 5 FIA 5* FIA 6 FIA 6* FIA 7 FIA 7* NA NA*

21.2  23.4  27.6  32.4  25.0 24.6 27.4 24.0 35.1  19.0 

22.4 20.5 26.7 25 31.5 29.2 35.2 33.8 27.6 26.4 28.6 27.0 35.2 34.0

27.9 27 27.4 

25.9 

35.3  29 28.8 29  36.5 

35.5 34.3 31.5 30.0 30.2 28.9

35.7 34.4 32.5 29.1 31.4 29.0

36.2 34 33.1  31.6 

36.2  33.6 30.2 32.7 31.6

36.5 35.8 33.7  33.4 33.0

36.8 

36.8 

37.1 

34 

34.3 33.7

38 34.2

39.6 35.9

FIA ¼ Fluid Inclusion Assemble; NA ¼ Not Assemble. * ¼ Reproduced Homogenization Temperature.  ¼ Failed to nucleate vapor bubbles in the second run.

of fluid inclusions by thermal re-equilibration. Our results do not show a relationship between inclusion size and homogenization temperatures (Fig. 6). Above lines of evidences suggest that our fluid inclusion samples are unaltered by thermal re-equilibration. Therefore, the homogenization temperature data of our study are reliable and representative of the temperature of salt precipitation. Moreover, reproduced homogenization temperatures are usually within a 2
C temperature-difference interval for the first and second run, consistent with previous studies (Roberts and Spencer, 1995; Lowenstein et al., 1998). All reproduced homogenization temperatures are lower than those of the first run, which is probably caused by damages of the fluid inclusion walls (Lowenstein et al., 1998). Thus, only homogenization temperature

data from the first run were used in our study as paleoclimate indictor. 5.2. Sea surface temperatures of the late Aptian Congo Basin Primary fluid inclusions in halite are usually contained in cumulate and chevron crystals. Cumulate crystals often precipitate at the airewater interface or within the upper water column and later sink to the bottom under the effect of gravity (Roberts and Spencer, 1995; Lowenstein et al., 1998). Chevron crystals are vertically oriented bottom-growth crystals which precipitate in shallow water (Roedder, 1984; Handford, 1990). Therefore, the homogenization temperatures of fluid inclusions from both

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91

Fig. 6. Histogram of homogenization temperatures plotted against size of inclusions.

cumulate crystals and chevron crystals record brine surface temperatures during halite deposition or approximate surface air temperatures in the case of shallow environments (Lowenstein and Hardie, 1985; Handford, 1990; Benison, 1995; Lowenstein et al., 1998; Zambito and Benison, 2013). Although all homogenization temperature data represent surface brine or shallow water temperatures, it cannot be expected that the complete temperature variation of surface brines will be captured based on homogenization temperatures of halite fluid inclusions no matter how many homogenization temperature data were analyzed and how large a sample is. This insufficient reconstruction of the full temperature range results from the fact that salts and contained fluid inclusions do not form at constant rates over the full daily and seasonal temperature variation range. Salt formation is usually higher during daytime, especially the afternoon, and during summer (Roberts and Spencer, 1995; Lowenstein et al., 1998; Benison and Goldstein, 1999). In addition, the formation of salt reflects not only temperatures but also factors such as humidity and wind velocity (Roedder, 1984; Roberts and Spencer, 1995). More importantly, only a limited number of the millions of fluid inclusions in an individual sample can artificially nucleate vapor bubbles, and thus, can be used for Th measurements. These observations imply that the ranges of homogenization temperatures may best represent the average temperature conditions during halite deposition (Roberts and Spencer, 1995). Therefore, we conclude that homogenization temperatures concentrated within 25e35
C in core ZK143 represent the sea surface temperature conditions of the Congo Basin, northern South Atlantic during the late Aptian. This result is in good agreement with previous outcomes of atmospheric general circulation models. Donnadieu et al. (2006) suggested that the mean annual surface temperature of this region during the Aptian was around 30
C. Similarly, climate models indicated that the warmest temperature during the Aptian was located over the subtropics of Gondwana which probably experienced a temperature range of 26e34
C (Fluteau et al., 2007). Thus, results from climate modeling analyses

support our fluid-inclusion based inferences and imply that our homogenization temperature data for the late Aptian Congo Basin are reasonable. However, analyses and observations of laboratory-grown and modern halite (Lowenstein et al., 1998; Meng et al., 2011b) demonstrated that maximum homogenization temperatures from fluid inclusions can be equivalent to the highest brine temperatures during halite crystallization. The highest homogenization temperature of 46.5
C (sample 3, 576.67 m) recorded in our study thus represents the highest sea surface temperatures of the Congo Basin during the late Aptian. Analogs of similarly high or even higher temperatures were reported from many modern and ancient evaporitic basins such as the Silurian Michigan Basin (59
C; Losey and Benison, 2000), the mid Cretaceous Khorat Plateau (62.1
C; Zhang et al., 2015) and the Quaternary Lop Nur of the Chinese Tarim Basin (58
C; Liu et al., 2006). Most of these basins represent continental settings and inferred maximum temperatures are thus higher than temperatures expected for maritime conditions (Hay and Floegel, 2012). Interestingly, large-scale potash deposits were formed in all these regions with extremely high temperature records. Consequently, Zhang et al. (2015) suggested that the largescale potash deposits of the geological past are partially related to high temperature or hot conditions. The recorded highest temperature from our study and its corresponding large-scale potash deposit in the central segment of the South Atlantic region support this hypothesis. In addition, the interpreted high temperature conditions in the South Atlantic can be indirectly inferred from the existence of tachyhydrite deposits. Tachyhydrite is rare in evaporitic basins due to its extremely high solubility. It is absent in modern Ca-rich brines even under very arid conditions (El Tabakh et al., 1999; Zhang et al., 2010). However, massive tachyhydrite deposits of more than 100 m thickness were formed along both margins of the South Atlantic region during the late Aptian often exhibiting primary textures (Wardlaw, 1972; de Ruiter, 1979). Experimental analyses demonstrated that tachyhydrite only remains stable when water temperatures exceed 35
C although its

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formation starts already in specifically composed sea waters (i.e. 92.7 mol CaCl2 and 49.2 mol MgCl2 per 1000 mol of water) at temperatures as low as 22
C (Wardlaw, 1972; Zhang et al., 2010). This observation implies that high temperatures likely support the formation of tachyhydrite. Modern and ancient analogs of tachyhydrite formation represent the Red Sea and the mid-Cretaceous Khorat Plateau where brines reached temperatures of up to 56
C and 62
C, respectively (Erickson and Simmons, 1996; Zhang et al., 2015). Thus, the broad occurrence of tachyhydrite in the South Atlantic during the late Aptian is tentatively regarded as additional evidence supporting our interpretation of homogenizationtemperature derived high sea surface temperatures. 5.3. The late Aptian climate cooling Although based on a low number of samples, climate change through late Aptian times is tentatively inferred from the stratigraphic pattern of the average and maximum homogenization temperatures (Fig. 7). The results show that sea surface temperatures underwent significant changes during the deposition of the Loeme Formation. Sea surface temperatures were relatively stable for the lower part of the formation with average homogenization temperatures of 29.5
C, 31.6
C and 31.0
C for samples 1e3 from 591.11 m, 579.21 m and 576.67 m, respectively. Maximum Th of 33.9
C, 39.6
C and 46.5
C suggest a dramatic increase over the same stratigraphic section. In contrast, climatic cooling is indicated

in the middle part (sample 4) at 388.97 m by both average and maximum Th of 28.2
C and 45.9
C (Fig. 7). Samples 5e6 from 287.34 m to 275.06 m suggest further cooling with average and maximum Th values of 26.4
C and 26.6
C, and 44.3
C and 42.0
C, respectively. Compared to the temperature data from the lower part of the Leome Formation, both average and maximum Th from samples 4e6 imply a sea surface temperature decrease of 4e5
C. Sample 7 from the upper part of the Leome Formation at 245.20 m apparently recorded a return to higher sea surface temperatures comparable to those inferred for the lower part, with average and maximum Th of 29.9
C and 41.9
C, respectively. Based on the resolution of available data, a detailed reconstruction of climatic conditions is not possible. However, the existing homogenization temperature data imply a late Aptian cooling of sea surface water temperatures by 4e5
C. In the southern Tethyan region, the late Aptian cooling was inferred from the increasing percentages of bisaccate pollen and the nannofossil Eprolithus floralis after the deposition of the Selli Level (Hochuli et al., 1999). Cooling at the early late Aptian and up to millions of years after the onset of the biocalcification crisis was also suggested by Weissert and Erba (2004) based on the pelagic bulk carbonate isotope record. Steuber et al. (2005) reported a cooling by 5
C during the late Aptian based on oxygen isotope analysis of Tethyan rudists. An equivalent cooling by about 5
C was also documented by the TEX86 proxy from the Mazagan Plateau in the eastern Atlantic off Morocco (McAnena et al., 2013). In addition,

Fig. 7. Paleoclimate trend indicated by homogenization temperatures.

H. Zhang et al. / Cretaceous Research 71 (2017) 85e95

the late Aptian was a time characterized by worldwide sea level lowstands which was documented on carbonate platforms of the Arabian region (Maurer et al., 2013), the Russian Platform (Sahagian et al., 1996), in western Siberia (Medvedev et al., 2011), western Canada (Zaitlin, 2002) and Europe (Heimhofer et al., 2007). These lowstands were recently attributed to the late Aptian cooling and explained by a glacio-eustatic mechanism (Maurer et al., 2013). The accumulating lines of evidence suggest that the late Aptian cooling has a global pattern. Therefore, the coeval decline of sea surface temperatures in South Atlantic region inferred from our analyses of homogenization temperatures of halite fluid inclusions agree with previously reconstructed global cooling during this interval. Although the late Aptian cooling has been documented by numerous geological records, its precise timing, duration and amplitude is still barely assessed. For example, McAnena et al. (2013) reported that sea surface temperatures of the Mazagan Plateau cooled by about 5
C during a duration of two million years. In contrast, long term cooling over about six million years was recorded in SW Iran and southern Italy (Maurer et al., 2013; Graziano and Raspini, 2015). However, the regional and global correlation of this cooling period remains difficult due to the different approaches for the established chronologies. Especially, in evaporitic settings where traditionally used dated proxies (e.g. ostracod, foraminifera, pollen, paleomagnetism) are severely limited due to the dominated huge salt deposits and environmental condition of extremely high salinity. A combination of higherresolution chronostratigraphic analyses, more detailed stableisotope records and correlations, and quantitative temperature reconstructions is required to achieve a better understanding of the late Aptian cooling in a regional-global framework. 6. Conclusions Homogenization temperatures of fluid inclusions of halite from the Loeme Formation in the Congo Basin have been used to quantitatively reconstruct the local sea surface temperatures during the late Aptian. Resulting homogenization temperatures mostly concentrated at 25e35
C represent prevailing sea surface temperatures of low-middle latitudes of the South Atlantic region and agree with previously reported results of climate modeling studies. The recorded maximum homogenization temperature of 46.5
C is similar to temperatures reported from modern and ancient evaporitic basins where large-scale potash deposits occur. Extremely high temperatures apparently supported the formation of extensive tachyhydrite deposits in the region. Significant cooling by about 5
C was identified by homogenization temperatures of halite fluid inclusions during the late Aptian. The comparison of our results with the coeval cooling reported from other regions of the world revealed that the cooling in the Congo Basin occurred in response to global cooling during the late Aptian although the precise timing, duration and amplitude of this cooling is still poorly constrained. Our study shows that sea surface temperatures derived from homogenization temperatures of halite fluid inclusions can provide robust and quantitative estimates of past climate conditions especially in evaporative settings where many other paleoclimate proxies are less appropriate. A detailed assessment of the global late Aptian climate conditions requires improved chronostratigraphical investigations and correlations combined with quantitative climate reconstructions. Acknowledgements This study was supported by the National Basic Research Program of China (973 Program) (No. 2011CB403000; No.

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Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10. 1016/j.cretres.2016.11.008.