Palaeoenvironmental and Climatic Changes during the Palaeocene-Eocene Thermal Maximum (PETM) at the Wadi Nukhul Section, Sinai, Egypt
Khozyem, Hassan, Adatte, Thierry, Spangenberg, Jorge E., Tantawy, Abdel Aziz, Keller, Gerta, Journal of the Geological Society
Abstract: The Palaeocene-Eocene Thermal Maximum (PETM) interval at the Wadi Nukhul section (Sinai, Egypt) is represented by a 10 cm thick condensed clay-rich layer corresponding to the NP9a-NP9b nannofossil subzone boundary. The Wadi Nukhul Palaeocene-Eocene boundary (PEB) is characterized by (1) an abrupt negative excursion in carbonate and organic carbon isotope ratios (-6[per thousand] in δ^sup 13^C^sub carb^ and -2[per thousand] δ^sup 13^C^sub org^), (2) an abrupt persistent negative shiftin organic nitrogen isotope values (δ^sup 15^N^sub org^), (3) a significant increase in phosphorus concentrations just above the carbon isotope excursion, (4) a decrease in carbonate content and significant increase in kaolinite and (5) high vanadium and low manganese contents coincident with the occurrence of framboidal pyrite. The abrupt correlative isotopic excursions of δ^sup 13^C^sub carb^, δ^sup 13^C^sub org^ and δ^sup 15^N suggest that the lowermost part of the PETM is missing. The decrease in carbonate content indicates dilution by high detrital input triggered by acid weathering and carbonate dissolution in response to increased atmospheric CO^sub 2^ resulting from the oxidation of methane. The sudden increase in kaolinite content reflects a short-lived change to humid conditions. The δ^sup 15^N values close to 0[per thousand] above the PEB suggest a bloom of N^sub 2^-fixing cyanobacteria. Increased bacterial activity may be either the cause or the result of the anoxia locally associated with the PETM.
The Palaeocene-Eocene boundary (PEB), 55.8 Ma ago (Westerhold et al. 2009), is a critical time interval in environmental changes and subsequent biotic changes. The global warming that started in the late Palaeocene and continued during the early Eocene known as the Palaeocene-Eocene Thermal Maximum (PETM), where the PEB transition interval shows exceptional global conditions, including rising temperatures to at least 8 °C at the poles and 5 °C in the tropics for several tens of thousands of years (Sluijs et al. 2006; Zachos et al. 2006; Weijers et al. 2007; Handley et al. 2008). These climatic changes coincide with a negative carbon isotope excursion (CIE) of -2[per thousand] in marine organic carbon (Sluijs et al. 2006) and -6[per thousand] in bulk marine carbonate (Zachos et al. 2005), and severe extinctions in benthic foraminifers (35-50%) (Alegret et al. 2009). Moreover, the widespread abundance of kaolinite in marine sediments during the PETM interval throughout the Tethys region points to a warm and humid late Palaeocene-early Eocene climate with high rainfall (Bolle et al. 2000; Bolle & Adatte 2001).
Catastrophic methane release from hydrates (clathrates) could be responsible for the rapid climatic change (Dickens et al. 1995; Kennett et al. 2002). Methane is stored along the continental margin where its stabilization as methane hydrate requires high pressure and relatively low temperature, but it may become unstable if the ocean warms rapidly (Dickens 2001). This could lead to the release of carbon estimated at 2000 × 109 tonnes over 10 ka (Zachos et al. 2005), which is commonly believed to be the main cause for the PETM (Katz et al. 2001; Dickens 2011). Many other hypotheses have been recently proposed by several researchers to explain this increase in atmospheric CO2, including the following: (1) the extensive burning of Palaeocene peat and coal deposits linked with the arid period that prevailed during the latest Palaeocene was suggested by Kurtz et al. (2003), but in their recent study, Moore & Kurtz (2008) did not find any indications of such a process in cores from either the Atlantic or the Pacific; (2) thermogenic methane linked to hydrothermal injection in organic-rich sediments leading to the explosive release of methane from Cretaceous-Palaeocene mudstones in the North Atlantic (Westerhold et al. 2009); this is unlikely for the Tethys area, which was tectonically stable during this period; (3) the drying of isolated epicontinental seas, leading to rapid oxidation of organic matter; (4) melting of the methane-rich permafrost (DeConto et al. 2010).
In Egypt, the base of the PETM is also characterized by minimal phosphorus accumulation rates, low biological productivity, a sealevel fall and increase in detrital content followed by a rapid increase in primary production and accelerated phosphorus accumulation (Bolle et al. 2000) coincident with increased humidity recorded by high kaolinite content.
This study examines the PETM interval in the Wadi Nukhul, Sinai, Egypt, focusing on climatic and palaeoenvironmental changes before, during and after the PETM spanning an interval of about 3.5 m. This interval is part of a 38 m thick succession of upper Maastrichtian to lower Eocene pelagic sediments exposed at Wadi Nukhul in the southwestern part of Sinai (29°02'06"N, 33°11'47"E) about 9 km east of Abu Zenima City (Fig. 1). Sediment deposition during the PETM interval occurred at about 500-600 m water depth, in an upper bathyal environment (Speijer et al. 1997).
Material and methods
A total of 25 samples covering the Upper Palaeocene-Lower Eocene interval were examined for bulk and clay mineralogy, major and trace elements, total phosphorus (Ptot), and stable isotopes (δ13Ccar, δ13Corg, δ15Norg). Carbon isotope analyses of aliquots of all samples were performed using a Thermo Fisher Scientific (Bremen, Germany) GasBench II preparation device interfaced to a Thermo Fisher Scientific Delta Plus XL continuous flow isotope ratio mass spectrometer. Analytical uncertainty (2σ) monitored by replicate analyses of the international calcite standard NBS-19 and the laboratory standards Carrara Marble and Binn Dolomite is no greater than ±0.05[per thousand] for δ13Ccarb.
Organic carbon and nitrogen isotope analyses were performed on decarbonated and oven-dried bulk sediment samples by flash combustion on a Carlo Erba 1108 elemental analyser connected to a Thermo Fisher Scientific Delta V isotope ratio mass spectrometer that was operated in the continuous helium flow. The δ13Corg and δ15Norg values are reported relative to VPDB and air-N2, respectively. The reproducibility was better than 0.1[per thousand] (1σ) for both carbon and nitrogen.
Bulk rock and clay mineralogy was determined by X-ray diffraction based on the method described by Kübler (1987) and Adatte et al. (1996) using a Thermo-Xtra system.
Elemental analyses were performed with an FrX Philips PW2400 X-ray fluorescence spectrometer using lithium tetraborate fused pellets for major elements and pressed powder pellets for trace elements. The detection limits are 0.01% for major elements and 1-4 ppm for trace elements. The accuracies of the analyses were assessed by analyses of standard reference materials.
Scanning electron microscopy (SEM; Tescan Mira/LMU) of gold- and carbon-coated PEB samples was used to assess the PEB interval for the presence of bacterially induced sedimentary structures (e.g. structures of extracellular polymeric substances associated with framboidal pyrite).
Total phosphorus (Ptot) analyses were performed for all Wadi Nukhul samples using the ascorbic acid method (Mort et al. 2007). replicate analyses indicated a precision better than 5%. In addition, Ptot analyses were performed on several other sections from the central Eastern Desert of Egypt (e.g. Gebel Duwi, Bolle et al. (2000); Dababiya Global Stratotype Section and Point (GSSP), Dupuis et al. 2003) and from central Sinai (Abu Zenima and Gebel Matulla, Bolle et al. 2000).
Biostratigraphy is based on calcareous nannofossils, which were analysed from smear-slides using a light microscope and 1000× magnification (Aswan University, Egypt).
At Wadi Nukhul, the PETM is part of the Esna Formation (5 m above the Tarawan Formation). The studied interval can be divided into three parts from the base to the top: (1) the 1.10 m thick grey-green marls of the uppermost Palaeocene; (2) the 80 cm thick alternating brown to grey marl with gypsum veins and anhydrite, starting at the base with a 10 cm thick condensed clay-rich interval including the PETM; (3) the 1.4 m thick grey-green marl of lower Eocene age. Following the recommendation of the International Commission of Stratigraphy (ICS), the PEB is defined by the basal inflection of the carbon isotopic excursion (δ13Ccarb). At Wadi Nukhul, the PEB is thus placed at the minimum of the δ13Ccarb excursions located at the base of the brown clay layer (sample 117, Figs 2 and 3).
Stable isotopes: δ13Ccarb, δ13Corg and δ15Norg
Wadi Nukhul carbon isotopes show similar trends in δ13Ccarb and δ13Corg values. Below the PETM (samples 110-116) δ13Ccarb and δ13Corg values average 1.1[per thousand] and 9.4[per thousand], respectively (Fig. 4). At the condensed clay-rich interval, the δ13Ccarb values decrease abruptly (c. -4[per thousand]) followed by a similar but delayed (5 cm upwards) shiftin δ13Corg (c. -28[per thousand]). Above the condensed clay-rich interval (samples 119-129) δ13Ccarb values increase gradually to the background values observed below the PETM, whereas low δ13Corg values persist to the end of the PETM interval. Above the PETM interval δ13Ccarb returns to pre-PETM values, whereas δ13Corg shows strong fluctuations (Fig. 4). A similar pattern is observed in δ15Norg values, although with increasing values below the PETM, followed by an abrupt decrease in δ15Norg values that persists through the PETM interval. Above this interval the δ15Norg values are more depleted but fluctuating, similar to δ13Corg (Fig. 4).
For the calcareous nannofossil biostratigraphy the modified standard zonal scheme of romein (1979) was used. The Palaeocene-Eocene interval is identified based on Discoaster multiradiatus, which defines zones NP9a and NP9b spanning the studied interval. Bybell & Self-Trail (1997) divided zone NP9 into a and b subzones based on the extinction of Fasciculithus species, such as F. hayi, F. clinatus, F. lillianae, F. bobii, F. alanii and F. mitreus. In the Wadi Nukhul section, these species disappear in the middle part of NP9 in a 10 cm thick condensed clay-rich interval that may be due to the dissolution of carbonate sediments and is coincident with a sharp decrease in Toweius spp., Coccolithus pelagicus/Ericsonia subpertusa (NP9a) and the carbon isotope excursion (Fig. 3). Subzone NP9b is marked by persistence of Toweius spp., blooms of Coccolithus pelagicus/Ericsonia subpertusa and the first appearance of short-lived Rhomboaster spp. and Discoaster araneus (the rD assemblage; Fig. 3).
Above the condensed clay-rich interval (sample 118), Toweius spp. gradually increase with a notable decrease in Coccolithus pelagicus/subpertusa marking a gradual return to pre-PETM environmental conditions.
The PETM interval (samples Nu-117-129) shows a 10 cm condensed clay-rich interval at the base with a rapid decrease in carbonate content to 5.14% and no change in the other bulk-rock components except anhydrite, which reaches maximum contents (17.08%) within the PEB, and phyllosilicates and quartz, which reach their maximum contents in the middle of the condensed clayrich interval (39.7% and 9.7% respectively) coinciding with low calcite content. At the top of the condensed clay-rich interval, Ca-apatite suddenly increases (to 17.22%). Above this condensed interval, clays grade into marls as calcite increases (mean value 61.81%) and phyllosilicates decrease (6.87%). Anhydrite disappears in the middle of the PETM interval. Above the PETM interval (samples Nu-130-135) sediments become more marly and all mineral components return to normal pre-PETM values (Fig. 5, see Fig. 6 for weathering indexes). In addition, microscopic observations indicate clear sedimentary structures related to bacterial activities, with characteristic shapes for cyanobacteria and sulphur-reducing bacteria associated with framboidal pyrite less than 5 µm in diameter (Fig. 7) within and close to the PEB samples (Nu-117, Nu-118 and Nu-119).
Clay minerals detected at Wadi Nukhul are kaolinite, smectite, illite, sepiolite and palygorskite. Kaolinite shows a gradual increase below the PETM (mean values 8.3%), rapidly increases to 50% just above the δ13Ccarb excursion and then returns to pre-PETM values (Fig. 5). Smectite sharply decreases (11.7%) below the PETM and remains low into the condensed clay-rich interval, followed by a gradual increase to values up to 80% that are commonly observed below and above the PETM in several sections located in Sinai (e.g. Mattula, Bolle et al. 2000). As palygorskite and sepiolite have the same origin and indicate the same environment, we have combined their values; they are present just below the PEB in high amounts (72.3%) but decrease during the PETM and gradually disappear above it (Fig. 5).
The kaolinite/smectite ratio is close to zero below and above the PEB, but shows an abrupt increase, up to 2.1, at the middle of the condensed clay-rich interval. The kaolinite/(sepiolite + palygorskite) ratio shows the same trend through the PEB but returns more gradually to background values above the PETM interval (sample 129).
Major and trace elements
Major and trace elements are normalized to aluminium (Tribovillard et al. 2006). The palaeo-depositional environment during the PETM is evaluated based on (1) elements indicative of detrital input rates (Si/Al, Ti/Al, Zr/Al and K/Al), (2) productivity-sensitive elements (Ptot, Ni/Al, Zn/Al and Cu/Al) and (3) redox-sensitive parameters Mn*, V/Al, and V/Cr ratio, where Mn* is calculated as log((MnSample/ MnPAAS)/(FeSample/FePAAS)) (Cullers 2002). The variations of different weathering indices were examined.
Below the PETM, Ti/Al, K/Al and Zr/Al ratios increase slightly from the base towards the PEB with average values of 0.084, 0.25 and 0.0037 respectively, whereas Si/Al contents decrease to 3.15 just below the PEB (Fig. 6). At the PEB, both Ti/Al and Zr/Al reach maxima (0.11 and 0.0045) and Si/Al and K/Al show minimum values (2.51 and 0.14). A notable increase in Si/Al, Ti/Al and K/Al is observed just above the PEB with maximum values in the middle PETM (6.24, 0.10 and 0.55 respectively), coeval with a drop in Zr/ Al to zero. Above the PETM interval these elements recover to pre-PETM values (Fig. 6).
The weathering index (WI) depends mainly on the mobility of major elements and their behaviour during the weathering process of the parent rock; some may increase with the progress of weathering whereas other decrease. For this reason we use three different weathering indices to emphasize the role of weathering during the PETM event.
(1) The Weathering Index of Parker, WIP = 100((2Na2O/0.35) + (MgO/0.7) + (2K2O/0.25)(CaO/0.7)), can be applied to weathered material that was derived from acidic, intermediate and basic igneous rocks taking into account the Al mobility (Parker 1970; Price & Velbel 2003). WIP values show a strong negative shift(2852) from mean values below and above the PEB (c. 6000) at Wadi Nukhul.
(2) The Chemical Index of Alteration, CIA = 100(Al2O3/ (Al2O3 + CaO* + Na2O + K2O)), measures feldspar conversion to clay minerals such as kaolinite, which explains the migration of climatic zones (Nesbitt & young 1984, 1989; Price & Velbel 2003). CIA values are high (53) across the PEB at Wadi Nukhul, which suggests intense chemical weathering.
(3) The Chemical Index of Weathering, CIW = 100(Al2O3/ (Al2O3 + CaO* + Na2O)) (Harnois 1988) is similar to CIA but eliminates K2O from the equation and reflects the progressive conversion of feldspar to clay.
CIW and CIA have identical values across the PEB at Wadi Nukhul, exceeding the background values only at the PEB (Fig. 6).
V/Al, V/Cr and Mn* show similar behaviour before and after the PETM. Below the PEB, these ratios are very low with average values 1.8 (10-4), 0.81 and 0.61 respectively. Mn* reaches minimum values (-0.49) at the base of the Eocene (sample 118, Fig. 7). V/Al shows the highest values at samples Nu-119 (52.6 × 10-4) and Nu-120 (155 × 10-4). V/Cr also reaches maximum values at the same level (1.21 for Nu-119, 2.54 for Nu-120), and gradually returns to the background value (Fig. 7).
The productivity parameters in this study are based on the element concentration ratios Cu/Al, Ni/Al and Zn/Al and Ptot, which are strongly related to organic matter. Below the PETM, all elements, except Ba, show low concentrations, with mean values of Zn/Al (1083 × 10-4), Cu/Al (22 × 10-4), Ni/Al (95 × 10-4) and Ptot (2790 ppm) slightly increasing to the top of the Palaeocene (Fig. 7). At the PEB all parameters show a sudden decrease to minimum values. Above the PEB to the middle PETM interval (samples from 119 to 124) all productivity-sensitive trace elements and Ptot reach maximum values (Cu/Al 231 × 10-4; Ni/Al 923 × 10-4; Ptot 5498 ppm). Above this interval the values for these elements return to the Palaeocene background concentrations (Fig. 7).
Completeness of the PETM at Wadi Nukhul
Whether a sedimentation record is continuous or partly missing because of a hiatus can be assessed by the abruptness of geochemical changes. At Wadi Nukhul, the negative δ13Corg and δ13Ccarb excursions are very abrupt (from -25.5 to -28[per thousand] and from 1 to -4[per thousand], respectively). Similar sharp negative excursions are observed in δ15Norg values (Fig. 4). Such abrupt shifts suggest the presence of a hiatus. Lithologically, the base of the Eocene at Wadi Nukhul shows a sudden change from marl to a condensed clay-rich layer that is characterized by an abrupt decrease in calcite content from 32.4% to 7.6% coinciding with a sharp increase in detrital components (phyllosilicates and quartz). At the same level, the absence of benthic foraminifera (Speijer & Wagner 2002) could be due to carbonate dissolution and/or dilution by increased detrital input. All these criteria together with the abrupt shiftin δ13Ccarb support the presence of a hiatus, which is also observed in other sections from Egypt (e.g. Gebel Duwi and Gebel Aweina sections; Speijer et al. 2000, 2002), Italy (Forada section, Agnini et al. 2007; Contessa road, Giusberti et al. 2009), Spain (Zumaya section; Schmitz et al. 1997), Uzbekistan (Aktumsuk section; Bolle et al. 2000), Ocean Drilling Program (ODP) sites 690, 685, 248 689, 865, 1051, 1263, 1260B and 1172D (Katz et al. 2003; Mutterlose et al. 2007) and Deep Sea Drilling Project (DSDP) site 401 (Pardo et al. 1997). A gradual decrease in carbon isotope ratios 1-2 m below the PEB (estimated to represent 120 ka) was observed at Alamedilla, Spain (Alegret et al. 2009), Aktumsuk, Uzbekistan (Bolle & Adatte 2001) and Dababiya, Egypt (Aubry et al. 2007) (Fig. 8).
Comparison of the carbon isotope result of the Wadi Nukhul section with the already published record of the Dababiya GSSP (Dupuis et al. 2003; Aubry et al. 2007) suggests that the lowermost Eocene sediments are missing from Wadi Nukhul (bed-1 to middle of bed-3 in the Dababiya GSSP and at ODP site 690B in the South Atlantic). This missing part of the section spans about 50.6 ka (Figs 2, 4 and 5) and corresponds to a third-order sea-level fall overlain by the next transgressive system tract (TST) coincident with the PETM (Ernst et al. 2006). This may also explain the absence of lowermost Eocene sediments from the Wadi Nukhul section.
Bulk-rock composition of the Palaeocene-Eocene interval at Wadi Nukhul reflects changes in the source and proximity of sediments from terrigenous areas and the intensity of erosion and transportation in response to sea-level and climatic changes. Below the PETM, most of the minerals analysed show normal background concentrations, except phyllosilicates, which increase slightly to the detriment of calcite (Fig. 5). This may be linked to a low sea level and proximity to a detrital source. The onset of the PETM is marked by an abrupt decrease in calcite coincident with a sharp increase in phyllosilicates and quartz (see detrital input (DI), Fig. 5). This increased detrital input reflects significant reworking linked to the early TST and/or intense dissolution owing to increased acidity (Zachos et al. 2005).
Moreover, low calcite is associated with increased Ca-apatite linked to increased biomass productivity, as indicated by Cu, Ni and Ba coeval enrichments. The presence of a single anhydrite layer at the same level remains difficult to explain but may be linked to methane hydrate release through complex redox cycles that led to sulphide oxidation (Arthur et al. 1988). Further investigations are needed to evaluate this process. Calcite content gradually returns to high values (70%) about 10 cm above the PEB and detrital input gradually decreases to normal background values. This reflects the normal rate of carbonate productivity and termination of the PETM event.
Clay minerals as palaeoclimatic indicators
Clay minerals and their relative abundance in marine sediments may record information on climate, eustasy, diagenesis or reworking (Chamley 1989). To interpret clay mineral associations, the relative roles of detritus and authigenesis must be distinguished. Smectite can be locally formed in the marine realm from early diagenesis, halmolysis (Karpoffet al. 1989), or hydrothermal weathering of volcanic rocks (Chamley 1998). Smectite may also be derived from alteration of ash layers. Finally, palygorskite and sepiolite are fibrous clay minerals that can be formed in situ as a result of hydrothermal processes as well as low-temperature alteration of Mg-bearing rocks (Karpoffet al. 1989). Most of the mechanisms explaining the authigenesis of palygorskite, sepiolite and smectite occur in hydrothermal environments such as mid-ocean ridges (e.g. Karpoffet al. 1989). However, the Wadi Nulkhul section is located in an upper bathyal setting (500-600 m depth, Speijer et al. 1997) and no evidence of such processes could be found. More commonly, clay minerals are of detrital origin and represent the end product of continental weathering and transport into marine basins. The nature of the clay depends mainly on the climatic conditions and the nature of the rock being weathered. Clay mineral assemblages therefore reflect continental morphology and tectonic activity, as well as climate evolution and associated sea-level fluctuations (Chamley 1989; Weaver 1989). Illite and chlorite are considered common by-products of weathering reactions with low hydrolysis typical of cool to temperate and/or dry climate. Smectite is formed in soils developed under a warm arid to temperate climate characterized by alternating humid and dry seasons (Chamley 1998). Palygorskite and, to a lesser extent, sepiolite are continental products frequently found in a lacustrine environment and calcrete soils in arid to semi-arid climatic zones. Nevertheless, palygorskite preferentially forms in perimarine environments where continental alkaline waters are concentrated by evaporation. This process is accelerated in higher temperature zones (Callen 1984; robert & Chamley 1991). Because each of the mechanisms for palygorskite formation requires warm and arid climatic conditions, its occurrence is considered as indicator of continental aridity (Chamley 1989; Adatte et al. 2002). Kaolinite is generally a by-product of highly hydrolytic weathering reactions in perennially warm humid climates.
The Wadi Nukhul sediments just below the PEB are enriched in palygorskite and sepiolite (15.8% and 56.5%, respectively) coinciding with high illite, which indicates that arid to semi-arid climate conditions prevailed just before the PETM (Bolle et al. 2000). The palygorskite and sepiolite amounts gradually decrease during the PETM and these minerals disappear in the Early Eocene samples (Fig. 5), where smectite content increases, indicating a more contrasted seasonal climate. The sharp increase in kaolinite observed in the lowermost PETM samples suggests a short period of warm humid conditions. The calculated kaolinite/smectite and kaolinite/(sepiolite + palygorskite) can be used as a signal of climatic change during the PETM interval. Both ratios reach their maximum during the PEB followed by a gradual decrease in the kaolinite/(sepiolite + palygorskite) ratio, which indicates a wet hot climate at the PEB followed by a gradual re-establishment of the semi-arid to arid conditions during the PETM.
Above the PETM, kaolinite/smectite and kaolinite/(sepiolite + palygorskite) values are close to zero, reflecting the prevalence of an arid climate after the PETM (Fig. 5). A similar kaolinite increase during the PETM, indicative of warm and humid conditions, has previously been observed in Sinai (Bolle et al. 2000), Spain (Lu et al. 1998), and Uzbekistan (Bolle & Adatte 2001). Dupuis et al. (2003) and Ernst et al. (2006) did not report the presence of palygorskite and sepiolite in the Dababya GSSP section, whereas Bolle et al. (2000) found only low amounts of palygorskite (up to 10%) in the nearby coeval sections of Abu Zenima and Gebel Matulla sections. These differences are probably due to the use of more accurate deconvolution software to discriminate the smectite and illite peaks from the palygorskite and sepiolite reflections.
Palaeoenvironmental geochemical proxies
The removal of dissolved major and trace elements from the water column and their incorporation into sediments depends on biotic and abiotic processes (Tribovillard et al. 2006). In oxic environments trace element enrichments are related to abiotic processes. In dysoxic-anoxic conditions, they are linked with diffusion and remobilization processes at the sediment-water interface through redox cycles in the sediments and microbial activity (Tribovillard et al. 2006), and more particularly with redox cycling of Mn and Fe (Lyons et al. 2003; Sageman & Lyons 2003; Algeo & Maynard 2004; riquier et al. 2006). Major and trace elements can thus provide insights into the palaeo-depositional environment of the PETM. For example, Al-normalized elements, such as Si, Ti, K and Zr, reflect enhanced delivery of detritus from different sources including riverine, aeolian or volcanic (Bertrand et al. 1996; Murphy et al. 2000; Pujol et al. 2006). High Ti/Al ratios may reflect enhanced detrital supply from rivers where Ti is associated with heavy mineral grains that require high transport energy from the source area and may therefore also indicate a sea-level fall. However, in some cases increased Ti correlates with a sea-level rise and thus reflects volcanic input, especially when high Ti input correlates with high Zr/Al ratios. In contrast, a uniform stratigraphic distribution of the K/Al, Fe/Al and Si/Al values suggests a homogeneous detrital supply (riquier et al. 2006).
At Wadi Nukhul, high Ti/Al and Zr/Al ratios coincide with high weathering indices, such as WIP, CIA and CIW (Figs 5 and 6). Increased weathering at the PEB may result in concentration of these elements owing to the removal of alkaline elements during weathering of the parent rock. The slight decrease in authigenic and/or biogenic Si at the PEB may be due to an increase in detrital input (Tribovillard et al. 2006), as is commonly observed in TST deposits.
Fluctuations of the bottom water oxygenation during depositional processes can be deciphered by using different elemental ratios (e.g. V/Cr, Ni/Co, U/Th, etc.) and Al-normalized contents of redox-sensitive or chalcophile elements (e.g. Fe, Mn, U, V, Zn, Pb, Cu, Ni) (Joachimski et al. 2002; Algeo & Maynard 2004; Cruse & Lyons 2004; Pujol et al. 2006). However, some of these elements are typically associated with hydrothermal activity (e.g. Pb, Zn, Cu, Co, Ag, Mo) and the cause of their enrichment remains often ambiguous (Cruse & Lyons, 2004). Because the Wadi Nukhul area was part of the southern Tethys margin (with palaeodepth about 500-600 m), which was tectonically stable during the late Palaeocene-early Eocene, these elemental ratios can be also used to constrain the water column oxygenation during the deposition of the PETM sediments.
The Mn* parameter is an indicator of redox conditions in depositional environments (Cullers 2002). In this study V/Cr was additionally used in combination with Al-normalized V and Mn* chalcophiles as redox sensitive to bottom water oxygenation (Fig. 7) in association with pyrite framboids. The calculated Mn* is the best way to represent the Mn part linked to redox conditions. We use this parameter because during diagenesis of Ca-rich sediments Mn(II) can replace Ca and form Mn-bearing carbonates.
Vanadium in oxic waters is present as V(IV) and V(V) ions, which are tightly coupled with the redox cycle of Mn. Vanadate readily adsorbs onto Mn- and Fe-oxyhydroxides and possibly kaolinite (Tribovillard et al. 2006). Vanadium can also be derived from sedimentary biomass, where it occurs in the active sites of some enzymes in nitrogen-fixing bacteria (Anbar 2004; Grosjean et al. 2004; Tribovillard et al. 2006). According to Hoffman et al. (1998), a V/Cr ratio of five corresponds to the limit between dysoxic and anoxic conditions, whereas lower V/Cr ratios generally indicate oxic or dysoxic conditions (Pujol et al. 2006).
At Wadi Nukhul, the vertical distribution of both V/Al and V/Cr values suggests oxic conditions, except in the 5 cm interval just above the PEB (Fig. 7), which is characterized by the highest values in V/Al and V/Cr, but still in the range of oxic or dysoxic conditions (V/Cr = 3.6) although slightly dysoxic. This is consistent with the vertical distributions of Mn and Fe with very low values in the interval of dysoxic conditions (5 cm above the PEB) and increasing upward to the top of the PETM. This may reflect an increase in water column oxygenation in the upper PETM. The positive Mn* values above and below the PEB suggest oxidizing conditions in contrast to the negative Mn* value recorded within the PEB level (-0.4), which can be linked to increased anoxia. Most of the iron tends to be incorporated in sulphides under reducing conditions, explaining the presence of pyrite framboids of less than 5 µm in diameter (riquier et al. 2006) at the PEB (samples 117 and 119), and their small size reflects slightly anoxic conditions in the water column. This is also supported by the presence of various bacterial communities that remove oxygen and reduce sulphate for precipitation of small pyrite framboids (Fig. 7). Their presence is in apparent contradiction to the observed V/Cr and Mn* ratios indicative of dysoxic to slightly anoxic conditions as also observed by Speijer et al. (1997) using lithological and biotic criteria. This discrepancy could be explained by the removal of these redox-sensitive elements by oxidation owing to recent weathering processes.
Phosphorus (P) is the most important element used by all living organisms for their metabolism. In marine environments, P controls primary production over a long time scale (Tyrell 1999). Phosphorus content in sediments is strongly linked with organic matter (OM) content and reflects high productivity (Tribovillard et al. 2006). Phosphorus accumulation in sediments derives from various sources, including phytoplankton, fish scales, bones or detrital reworked P grains. In general, P is released as PO4 3- from decaying organic matter during oxic, suboxic and anoxic bacterial degradation below the sediment-water interface (Tribovillard et al. 2006). However, under anoxic conditions, P is not retained in the sediment and generally diffuses upward from the sediment and returns to the water column (Mort et al. 2008). Phosphorus enrichment may result from high OM supply in highly productive marine environments such as upwelling areas. In low-productivity areas P enrichment can be triggered through redox cycling of iron, with P adsorption onto ironoxyhydroxide coatings and Fe-P co-precipitation (e.g. Jarvis et al. 1994; Piper & Perkins 2004). An alternative mechanism for P retention may be uptake by bacterially precipitated polyphosphates. The degree to which remineralized organic P is retained as a reactive fraction in sediments depends on the redox conditions of the depositional system. In environments with at least intermittently oxic bottom waters, redox cycling of Fe in the sediments limits the diffusive flux of demineralized P to the overlying water column (Tribovillard et al. 2006; Mort et al. 2008).
At Wadi Nukhul, increasing relative abundances of Discoaster, Fasciculithus tympaniform and Sphenolithus primus have been interpreted to represent more oligotrophic, but also warmer environments as noted at ODP sites 690 and 1209 by Bralower (2002) and Gibbs et al. (2006). rD taxa are interpreted as proxies for warm surface waters and/or increased salinity with possibly low pH and increasing acidity (Tantawy 2006). Within the PETM an increase of both eutrophic Coccolithus and Ericsonia associated with decrease of Sphenolithus was interpreted by Agnini et al. (2007) as a response to nutrient-enhanced conditions induced by an increase in weathering and runoff. Above the CIE-PETM, the abundance shifts of nannofossil taxa that are considered indicators of cold-water conditions and/or that were adapted to mesotrophic-eutrophic environments, such as Toweius, could indicate the termination of the PETM climatic changes. Thus the Ptot fluctuations can be divided into four parts: (1) in the latest Palaeocene Ptot increases to exceed the mean value in normal oxic conditions; (2) at the PEB, Ptot depletion is at its maximum and coincides with anoxic oligotrophic conditions; (3) the highest concentration at 10 cm above the PEB is possibly due to an increase in eutrophic conditions and primary productivity and/or deposition of fish remains and coprolites within the laminated marl (Speijer & Wanger 2002; Soliman et al. 2006); such fish remains and coprolite accumulations typify condensed intervals characterizing the TST; (4) the decrease to the normal background above the lower PETM is due to the predominantly mesotrophic-eutrophic conditions.
Similar sequences of Ptot depletion and enrichment characterize the PETM interval in several sections in Egypt (Dababiya, Abu Zenima, Gebel Mattula and Duwi sections, Fig. 9). In all these sections, a significant decrease in Ptot contents coincides with the PEB, linked to anoxia that prevailed during eutrophic conditions, and followed by a sharp increase reflecting rising primary productivity and/ or deposition of condensed layers enriched in coprolite and fish remains. The correlation of these sections (Fig. 9) shows that the thickness of the Ptot-enriched interval and the Ptot contents decrease from proximal (Duwi and Dababiya) to more distal environments (Mutalla, Abou Zenima and Wadi Nukhul); this can be explained by reduced sedimentation in the deeper sections during the transgressive interval and/or higher Ptot input in shallower environments.
In oxic marine environments Ni, Zn and Cu act as nutrients and may be present as soluble Ni(II), Zn(II) and Cu(II) cations or chlorides. Ni is present as a soluble Ni-carbonate (NiCO3), or associated with Zn and Cu adsorbed into organic matter (Calvert & Pedersen 1996; Algeo & Maynard 2004; Tribovillard et al. 2006). These elements may also be adsorbed onto particulate Fe-Mn-oxyhydroxides. The formation of Ni, Zn and Cu complex compounds with OM accelerates removal from the water column and enrichment of sediments (Piper & Perkins 2004). At Wadi Nukhul, these elements show the same enrichment trends with high values 10 cm above the PEB (Fig. 7), coeval with increasing phosphorus and carbonate contents. Moreover, elevated Ni/Al, Zn/Al and Cu/Al ratios suggest high organic matter content, although organic matter is largely oxidized or removed as a result of strong weathering (Speijer & Wagner 2002). However, increased productivity is apparent above the δ13Corg negative excursion, reflecting a gradual recovery.
Detailed geochemical analyses of the Wadi Nukhul section in the SW Sinai and comparison with other sections in Egypt reveal the climatic and environmental changes across the Palaeocene-Eocene transition and PETM interval. The PEB at the Wadi Nukhul section shows the following features: (1) an abrupt negative shiftin δ13Ccarb followed by δ13Corg shift(-6[per thousand] and -2[per thousand] respectively), which suggests a hiatus including the earliest Eocene; this short hiatus is observed in the basal part of the PETM at Wadi Nukhul and also in most of the PETM section in Egypt except at the Dababiya GSSP; (2) a strong and persistent decrease in δ15Norg to c. 0[per thousand], reflecting significant bacterial activity such as sulphur-reducing and cyanobacteria activity as indicated by low N isotope and the presence of extracellular polymeric substances (EPS) like structures within the PETM interval; (3) a significant increase in Ptot content just above the negative isotopes excursion, which indicates increased primary productivity induced by increasing nutrients input linked with intense weathering period and prevalence of oligotrophic conditions; (4) a decrease in carbonate content, which may be linked to the dissolution and/or the dilution of carbonate owing to high detrital input; (5) the presence of small framboidal pyrite grains (less than 5 µm) coincident with high V content and negative Mn*; these features are strong evidence of increasingly dysoxic to suboxic conditions; (6) the topmost Palaeocene experienced a period of aridity, as indicated by the presence of both sepiolite and palygorskite, followed by significant increase in kaolinite contents triggered by humid climatic conditions, then a return to semi-arid conditions in the topmost CIE of the PETM interval.
We thank T. Monnier and J. C. Lavanchy for technical support and for carrying out phosphorus and X-ray fluorescence analyses. Also, we thank P. Vonlanthen for his help during SEM analyses, and A. Villars for the preparation of thin sections. We are deeply grateful to M. P. Bolle for providing us with samples from Gebel Mattula Abu Zenima and Duwi. We warmly thank B. Bomou for his precious help with fieldwork. We gratefully acknowledge K. Föllmi for stimulating discussion during the preparation of this paper. We thank r. Speijer and an anonymous reviewer for their constructive and thorough comments. This work was funded by the Egyptian Ministry of Higher Education (Mission No. 001/013/104) and from US-NSF OISE-0912144.
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received 4 April 2012; revised typescript accepted 24 September 2012.
Scientific editing by Quentin Crowley.
HASSAN KHOZyEM1,2*, THIErry ADATTE1, JOrGE E. SPANGENBErG1, ABDEL AZIZ TANTAWy2 & GErTA KELLEr3
1Present address: Institut de Science de la Terre (ISTE), Université de Lausanne, Lausanne, Switzerland
2Department of Geology, Aswan University, 81528, Aswan, Egypt
3Department of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08544, USA
*Corresponding author (e-mail: HassanMohamed.Saleh@unil.ch)…
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Publication information: Article title: Palaeoenvironmental and Climatic Changes during the Palaeocene-Eocene Thermal Maximum (PETM) at the Wadi Nukhul Section, Sinai, Egypt. Contributors: Khozyem, Hassan - Author, Adatte, Thierry - Author, Spangenberg, Jorge E. - Author, Tantawy, Abdel Aziz - Author, Keller, Gerta - Author. Journal title: Journal of the Geological Society. Volume: 170. Issue: 2 Publication date: March 2013. Page number: 341+. © Geological Society Publishing House Jan 2009. Provided by ProQuest LLC. All Rights Reserved.
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