Site description

In the Arabian Sea (north-western Indian Ocean; Fig. 3), the climate is controlled by the monsoon regime (Kutzbach, Reference Kutzbach1981; Prell, Reference Prell, Berger, Imbrie, Hayse, Kukla, Saltzman and Reidel1984). The monsoon phenomenon corresponds to a seasonal reversal of wind direction. Driven by the land–sea thermal contrast, monsoons affect the annual weather cycle between 30°N and 30°S. The south-west summer monsoon provides strong and wet winds, whereas the north-east winter monsoon brings weak and dry winds. The Owen Ridge is a key site in the north-west Indian Ocean for observing the onset and behaviour of the monsoonal circulation pattern using clay proxies. Indeed, the site is characterized by two contrasting sources in terms of clay mineralogy. During the summer monsoon, the south-westerly winds sweep the saline desert area of Somalia along the East African coast and Arabian Peninsula, bringing significant amounts of palygorskite (Debrabant et al., Reference Debrabant, Krissek, Bouquillon, Chamley, Prell, Niitsuma, Emeis, Al-Sulaiman, Al-Tobbah and Anderson1991; Krissek & Clemens, Reference Krissek, Clemens, Prell, Niitsuma, Emeis, Al-Sulaiman, Al-Tobbah and Anderson1991). Nair et al. (Reference Nair, Ittekkot, Manganini, Ramaswamy, Haake and Degens1989) estimated that during the summer 80% of detrital fluxes were delivered to the Owen Ridge by the south-western monsoon winds. In winter, the north-east monsoon is weak. However, south-central Asia still receives rainfall draining the Himalayan highlands. Illite released by physical erosion from the Himalayas is transported by the Indus River towards the Owen Ridge (Fagel et al., Reference Fagel, Debrabant, De Menocal and Demoulin1992b). Since the Middle Miocene, the ridge was uplifted above the level of active Indus-derived turbiditic flows (Debrabant et al., Reference Debrabant, Krissek, Bouquillon, Chamley, Prell, Niitsuma, Emeis, Al-Sulaiman, Al-Tobbah and Anderson1991). Following the uplift, the clay minerals are delivered to the Owen Ridge mainly by aeolian transport (from the west and south) and oceanic currents (from the north). The continuous hemipelagic sedimentation (35 m/Myr) on the Owen Ridge was expected to record a climatic influence that was masked in the adjacent deep-sea fans.

Figure 3. Location of coring sites in the Arabian Sea, Bay of Bengal and Central Indian Basin in the Indian Ocean, including DSDP (underlined number), ODP (plain number) and Marion Dufresne (MD) drilling campaigns. The arrows represent the main transport of clay minerals to the adjacent basins. The orange arrows indicate the south-western summer monsoon and associated north-western trade winds that transport palygorskite-rich dust to the Owen Ridge. Illite (and chlorite to a lesser extent) is delivered by fluviatile transport (Indus or Ganges). Smectite, mainly originating from the Indo-Gangetic Plain, is delivered by fluviatile transport to the Bay of Bengal and Central Indian Basin.

Site description

Proximal environments on the ocean floor such as fans and deltas have been widely studied to trace the origins of detrital supplies. The clay assemblages of those proximal environments are under the direct control of the adjacent continental landmasses. For instance, the Bengal Fan (Fig. 3), the largest submarine fan in the world (Curray et al., Reference Curray, Emmel and Moore2003), receives both the physical weathering products of Himalayan highlands (i.e. primary minerals illite and chlorite) and the chemical weathering products of the Indo-Gangetic Plain soils (i.e. secondary minerals, mainly smectite; Srivastava et al., Reference Srivastava, Parkash and Pal1998) via fluviatile transport by the Ganges and Brahmaputra rivers (Venkatarathnam & Biscaye, Reference Venkatarathnam and Biscaye1973; Kolla & Rao, Reference Kolla and Rao1990). Since the Bengal Fan sediments are very sensitive to monsoon rainfall, this study allows us to determine the relationships between regional climate and continental erosion (e.g. Joussain et al., Reference Joussain, Colin, Liu, Meynadier, Fournier and Fauquembergue2016).

Clay mineral variability was investigated in several marine cores at both proximal (e.g. MD77-180 (Colin et al., Reference Colin, Turpin, Bertaux, Desprairies and Kissel1999); BoB-56 (Li et al., Reference Li, Liu, Shi, Zhang, Fang and Chen2018)) and distal Bengal Fan sites (e.g. DSDP 218 (Bouquillon et al., Reference Bouquillon, Chamley and Frohlich1989); ODP Leg 116 Site 717C (Bouquillon et al., Reference Bouquillon, France-Lanord, Michard, Tiercelin, Cochran, Stow, Auroux, Amano, Balson and Boulègue1990; Brass & Raman, Reference Brass, Raman, Cochran, Stow, Auroux, Amano, Balson and Boulègue1990; Aoki et al., Reference Aoki, Kohyama and lshizuka1991); Fig. 3). In the <2 μm fraction, the clay mineral assemblages are characterized by changes in the relative abundance of illite and smectite over long timescales, from the early Miocene to Pleistocene. This mineral variability is explained either by changes in relative sedimentary inputs from Himalayan and Indian sources and/or changes in weathering intensity in the Indo-Gangetic Plain (e.g. Bouquillon et al., Reference Bouquillon, France-Lanord, Michard, Tiercelin, Cochran, Stow, Auroux, Amano, Balson and Boulègue1990; Aoki et al., Reference Aoki, Kohyama and lshizuka1991; France-Lanord et al., Reference France-Lanord, Derry and Michard1993; Derry & France-Lanord, Reference Derry and France-Lanord1996).

Materials and methods

Joussain et al. (Reference Joussain, Colin, Liu, Meynadier, Fournier and Fauquembergue2016) analysed the clay mineral assemblages in core MD12-3412 over the last 180 kyr with a millennial resolution. The 32 m-long core MD12-3412 (17°10’94”N, 89°28’92”E) was retrieved in the upper part of the Bengal Fan at a water depth of 2368 m near the continental slope (north-eastern Bay of Bengal; Fig. 3). Its hemipelagic sediments consisted of intercalations of clayey and silty layers interrupted by gravity-flow depositions (i.e. turbidites) related to changes in the regime of Ganges discharges. The δ¹⁸O vs depth curve from planktonic foraminifera, combined with seven radiocarbon ages, was correlated with the reference marine oxygen isotope composition vs time curve SPECMAP (Martinson et al., Reference Martinson, Pisias, Hays, Imbrie, Moore and Shackleton1987) to establish an age model for core MD12-3412, which produced an estimate of a high sedimentation rate ranging between 3 and 11 cm kyr^–1.

Only the upper 13.5 m of core MD12-3412 (i.e. ~180 kyr) was sampled at 5–10 cm intervals for clay mineralogical analyses (n = 240). Oriented mounts of carbonate-free <2 μm fractions were investigated using XRD (Holtzapffel, Reference Holtzapffel1985). The analysis was performed using a PANalytical X'Pert Pro Diffractometer equipped with a copper anticathode (Cu-Kα) and a Ni filter, under a voltage of 40 kV and a current intensity of 25 mA. The samples were measured with a counting time of 1 s/step in the range 3–30°2θ. The relative abundance of the main clay minerals was derived from the measurement of peak areas of the basal reflection (10 Å for illite, 7 Å for chlorite and kaolinite, 15–17 Å for smectite) on glycolated mounts using MacDiff software (Petschick, Reference Petschick1997). The relative proportions of kaolinite and chlorite were estimated using the (002) peak of kaolinite and the (004) peak of chlorite (i.e. 3.57 Å/3.54 Å; Biscaye, Reference Biscaye1965).

Site description

Major tectonic, climatic and oceanic current changes have occurred in the Indian Ocean since the early Cenozoic in relation to the Alpine orogenesis (e.g. Kennett, Reference Kennett1982). Such changes were expected to be recorded in the Indian Ocean sediments.

Materials and methods

Five long cores (i.e. MD90-947 to MD90-942, 33–49 m long) were recovered along a north–south (1–10°S) transect at ~80°E in the Central Indian Basin (Fig. 3) during the MD90/SHIVA cruise (Fagel et al., Reference Fagel, Debrabant and André1994). Two main lithologies were observed: a lower sedimentary unit made of reduced silty to sandy mud and an upper unit composed of oxidized siliceous clayey mud. According to radiolarian-derived biostratigraphy, the age of the sediments ranged from the Late Miocene to the Late Pliocene (<3.5/3.7 to >6.3 Myr).

The clay mineralogy was studied by XRD of the <2 μm carbonate-free fraction (Holtzapffel, Reference Holtzapffel1985), with a sampling interval of 30–40 cm (i.e. ~600 samples). A Philips PW 1730 diffractometer equipped with a copper anticathode and Ni filter was used with a voltage of 40 kV and an intensity of 25 mA. Analytical uncertainties are estimated as ±5% for clay mineral abundances >20% (Holtzapffel, Reference Holtzapffel1985).

In addition, the clay mineral variability was studied at high resolution in the cores MD90-946 (3°S) and MD90-943 (8°S) over three biostratigraphy-constrained intervals: specifically, between 5.7/5.8 and 6.3/6.5 Myr (Late Miocene) and between 1.2/1.5 and 4.3/4.7 Myr (Late Pliocene) for MD90-946 and between 5.0 and 5.6 Myr (Early Pliocene) for MD90-943. A total of 400 supplementary samples were analyzed by XRD to reach a resolution of 1 sample per 10 cm. The chosen clay proxy was the smectite/illite peak height ratio (S/I), which was measured on the glycolated diffractograms (Fagel et al., Reference Fagel, Debrabant and André1994). The fluctuations of S/I ratios through core depth were reported as a function of time by applying a mass accumulation rate estimated from stratigraphic information. Then, the time series were further studied by spectral analysis to detect any periodicity in the clay mineralogy.

Site description

The South China Sea (SCS; Fig. 6) is the largest marginal sea in the Pacific Ocean. Its sedimentation reflects the complex interactions between surface oceanic current patterns, East Asian monsoon winds and subsurface and deep currents from the adjacent Pacific Ocean intruding from the south (Wang et al., Reference Wang, Clemens and Liu2003). The East Asian monsoon, a major component of the global climate system, results from differential heating of the Asian continent and the western Pacific Ocean, which causes large seasonal contrasts in wind, precipitation and surface currents (Webster, Reference Webster, Fein and Stephens1987). In the summer, heating of the Asian continent generates a low-atmospheric pressure cell over central China, which induces wind to blow from south-west to north-east, bringing heavy monsoon rainfall over the south-eastern Asian continent (Webster, Reference Webster, Fein and Stephens1987). Conversely, in winter, a high-pressure cell over northern Asia related to the low temperatures over the Asian continent is responsible for dry and cold winds blowing from continental Asia to SCS in the south-west direction (Fig. 6a). In addition to local monsoon-related climatic changes, the SCS also records global glacial/interglacial oscillations (Boulay et al., Reference Boulay, Colin, Trentesaux, Frank and Liu2005). During glacial period, the SCS became a semi-enclosed marginal sea due to the significant sea-level drop (≥100 m; Fig. 6b). The modified glacial coastline was characterized by a clockwise surface circulation gyre in the summer and an anticlockwise gyre in the winter (Wang & Wang, Reference Wang and Wang1990).

Figure 6. Location of ODP coring sites 1145 and 1146 in the SCS (modified from Liu et al., Reference Liu, Trentesaux, Clemens, Colin, Wang, Huang and Boulay2003). The small coloured arrows indicate the provenance of clay minerals, with smectite-rich supplies in blue, illite in green, chlorite in grey and kaolinite in red. The lengths of the arrows indicate the relative importance of the clay mineral contributions from the various sources. (a) Main sources of clay minerals during interglacial periods. The winter north-eastern winds mainly brought illite and chlorite that were delivered by the Yangtze River (large green–grey arrow). The summer south-western winds transport smectite produced by weathering of volcanic material (large blue arrow). The surface oceanic currents then distribute the clay minerals across the SCS (black arrows). (b) Modifications occurring during glacial periods when the SCS became a semi-enclosed basin.

The delivery of clay minerals in the surface sediments of the SCS (Fig. 6) was controlled by the river discharge of weathering products from the adjacent Asian continent (Chen, Reference Chen1978). Approximately 570 Mt yr^–1 of suspended sediment (2.8% of global discharge to the world oceans; Milliman & Syvitski, Reference Milliman and Syvitski1992) are delivered to the SCS from three main sources (the Mekong River, the Pearl River and the Red River) and small rivers in south-western Taiwan (Liu et al., Reference Liu, Colin, Li, Zhao, Tuo and Chen2010). The north-eastern SCS receives 46% of this discharge (260 Mt yr^–1) from the Pearl River, south-western Taiwan and the Luzon arc system (Liu et al., Reference Liu, Colin, Li, Zhao, Tuo and Chen2010).

The distribution of clay minerals at the surface sediments of the SCS is mainly controlled by mineral provenance, with six mineralogical provinces being differentiated (Chen, Reference Chen1978):

1. (1) Illite and chlorite are the dominant minerals in the northern shelf of the SCS, the Taiwan Strait and the East China Sea. They are delivered by deep-water currents to the northern SCS by mountainous rivers draining the island of Taiwan and by the Yangtze River through the Taiwan Strait, with minor aeolian contributions from northern Asia due to north-eastern winter monsoon winds.

2. (2) Smectite and kaolinite are somewhat greater in content (~7%) relative to illite and chlorite in the central part of the SCS and north-east Luzon Strait.

3. (3) Smectite is >20% around the volcanic arc of the island of Luzon. Smectite-rich supplies are further transported to the northern SCS by surface currents influenced by the Kuroshio current originating from the adjacent Pacific Ocean.

4. (4) Smectite reaches ~30% in the southern SCS along Malaysia and Borneo. They are supplied by the south-western summer monsoon winds and transported by the surface oceanic currents to the northern end of the SCS and to the East China Sea through the Taiwan Strait (Fig. 6a; Liu et al., Reference Liu, Trentesaux, Clemens, Colin, Wang, Huang and Boulay2003, Reference Liu, Colin, Li, Zhao, Tuo and Chen2010).

5. (5) Kaolinite represents 50% of the clay mineral assemblage in the estuary of the Pearl River, then decreases downslope, probably due to rapid settling of coarse particles.

6. (6) Illite and chlorite are abundant in the estuary of the Mekong River (47% and 23%, respectively).

The relative contributions of clay minerals in the SCS sedimentary record were affected by the modified morphology of the SCS during glacial periods due to a sea-level drop of ~100 m (Fig. 6b). The SCS became a semi-enclosed basin characterized by an anticlockwise surface circulation driven by the winter monsoon winds (Wang et al., Reference Wang, Wang, Bian and Jian1995). More supplies of kaolinite were delivered to the coring site by the Palaeo-Pearl River due to the significant south-eastern shift of its estuary. Illite was mainly supplied from the island of Taiwan, whereas smectite was delivered to the south SCS by the Palaeo-Sanda River and to the north-eastern SCS from the island of Luzon. The distribution of clay minerals in SCS sediments may be used to trace the provenance and to identify the main transport agents supplying the detrital clay minerals to the marginal oceanic basins (Lui et al., Reference Liu, Trentesaux, Clemens, Colin, Wang, Huang and Boulay2003, Reference Liu, Colin, Li, Zhao, Tuo and Chen2010).

Materials and methods

ODP site 1146 (19°27.40’N, 116°16.37’E) was drilled on the continental slope of the northern SCS, at a water depth of 2092 m and near to the Pearl River estuary (Fig. 6). The sediments consisted of nannofossil-containing clays. The samples (n = 515) were retrieved from the upper 190 m of holes 1146A, B and C. The oxygen isotope record measured using foraminifera Globigerinoides ruber (surface-dwelling planktonic species) of ODP Site 1146 was correlated to the reference δ¹⁸O curve of ODP Site 677 in the east Atlantic (1°12’N, 83°44’W, 3461 m water depth; Shackleton et al., Reference Shackleton, Berger and Peltier1990). The studied interval covered the last 2 Myr. According to the age-depth model, derived from a combination of oxygen isotope stratigraphy, biostratigraphy and palaeomagnetism, the sampling interval of ~40 cm provided an average temporal resolution of 4 kyr (Liu et al., Reference Liu, Trentesaux, Clemens, Colin, Wang, Huang and Boulay2003).

Clay minerals were identified by XRD on oriented mounts of carbonate-free, clay-sized particles (<2 μm) according to the methodology of Holtzapffel (Reference Holtzapffel1985). The analysis was performed using a Philips PW 1710 diffractometer with Cu-Kα radiation and Ni filter, with a voltage of 40 kV and a current intensity of 25 mA (Trentesaux et al., Reference Trentesaux, Liu, Colin, Boulay, Wang, Prell, Wang, Blum, Rea and Clemens2003). Three analyses were performed per sample for air-dried and glycolated conditions in the 2.5–32.5°2θ range and from 2.5 to 14.5°2θ for heated samples (2 h at 490°C). Semi-quantitative estimates were derived from measurements of the glycolated mounts using the peak areas of the following peaks: smectite at 17 Å, mixed-layer illite–smectite at 15 Å, illite at 10 Å and kaolinite/chlorite at 7 Å, using the MacDiff software (Petschick, Reference Petschick1997). The clay mineral ratio (smectite + illite–smectite)/(illite + chlorite) was calculated from the peak areas to evaluate the controls over clay mineral variations over the last 2 Myr (Liu et al., Reference Liu, Trentesaux, Clemens, Colin, Wang, Huang and Boulay2003). A spectral analysis was performed on the evolution of the clay mineral ratio (smectite + illite–smectite)/(illite + chlorite) to reveal any periodicity in the time series covering the last 2 Myr.

Site description

The northern North Atlantic basins play a key role in the formation of the North Atlantic Deep Water (NADW; Fig. 7), an essential component of the global thermohaline circulation that controls interhemispheric heat exchanges (Broecker & Denton, Reference Broecker and Denton1989). The NADW results from a combination of three components: the Northeast Atlantic Deep Water (NEADW), the Denmark Strait Overflow Water (DSOW) and the Davis Strait Overflow (DSO; Dickson & Brown, Reference Dickson and Brown1994; Lucotte & Hillaire-Marcel, Reference Lucotte and Hillaire-Marcel1994). The NEADW and DSOW are driven in an anticlockwise gyre from the southern tip of Greenland to the outlet of the Labrador Sea by the Western Boundary Undercurrent (WBUC; McCartney, Reference McCartney1992). The NADW, produced offshore from Newfoundland, first flows southwards, then mixes with Circumpolar Deep Water in the Southern Hemisphere and returns back to northern latitudes through the Indian and Pacific oceans (Broecker, Reference Broecker1991). Numerous studies have shown variable intensity of the NADW over glacial and interglacial cycles (e.g. Ledbetter & Balsam, Reference Ledbetter and Balsam1985; Boyle, Reference Boyle1995; Barker et al., Reference Barker, Knorr, Vautravers, Diz and Skinner2010), but also over shorter (millennial) timescales (e.g. Bond et al., Reference Bond, Kromer, Beer, Muscheler, Evans and Showers2001; Oppo et al., Reference Oppo, McManus and Cullen2003).

Figure 7. Locations of studied coring sites in the Labrador Sea. The deep circulation patterns in the northern North Atlantic basin are adapted from Dickson & Brown (Reference Dickson and Brown1994) and Lucotte & Hillaire-Marcel (Reference Lucotte and Hillaire-Marcel1994). The light grey lines indicate water depth. DSO = Davis Strait Overflow; DSOW = Denmark Strait Overflow Water; ISOW = Iceland Sea Overflow Water; NADW = North Atlantic Deep Water; NAMOC = North-West Atlantic Mid-Ocean Channel; NEADW = North-East Atlantic Deep Water; NWADW = North-West Atlantic Deep Water; WBUC = Western Boundary Undercurrent.

In the deep North Atlantic basins, clay minerals are mainly detrital, derived from the weathering of adjacent continental masses (Biscaye, Reference Biscaye1965; Piper & Slatt, Reference Piper and Slatt1977; Zimmermann, Reference Zimmerman1982). The clay mineral assemblages in surface sediments of northern North Atlantic basins display a clear spatial distribution (Fagel et al., Reference Fagel, Robert and Hillaire-Marcel1996). The greatest abundances of smectite were observed in the Iceland (>60%; Fig. 7) and Irminger (57%; Fig. 7) basins. In the Labrador Sea, illite and chlorite were the most common clay minerals on the Canadian margin (29–39% and 21–29% respectively). They were associated with up to 30% vermiculite at shallow depths (≤530 m) but only traces of vermiculite at greater depths (>2650 m). Smectite, which was absent in the shallowest sites of the Canadian margins, occurred at greater depths (>2650 m) along both Labrador Sea margins. The greatest smectite abundance was measured on the continental rise of both Labrador Sea margins, within a water depth of 2800–3400 m, a depth interval consistent with the axis of maximum velocity of the WBUC. As a supply from the adjacent Greenland and Canadian margins is improbable, the WBUC is most probably responsible for the transport of smectite-rich material from the eastern Iceland and Irminger basins and for their sedimentation in the Labrador Sea (Fagel et al., Reference Fagel, Robert and Hillaire-Marcel1996). This observation shows that clay minerals may be used as proxies for deep circulation. The clay mineral variability in Labrador Sea sediments was therefore used as a proxy for deep-water sediment transport over glacial/interglacial cycles (Fagel et al., Reference Fagel, Hillaire-Marcel and Robert1997b; Fagel & Hillaire-Marcel, Reference Fagel and Hillaire-Marcel2006).

Materials and methods

Three cores drilled in the Labrador Sea were analysed. ODP Site 646 (58012.26’N, 48°22.15’W) was located off Greenland, on the upper flank of the Eirik Ridge sediment drift, below the axis of maximum velocity of the WBUC (Fig. 7; Fagel & Hillaire-Marcel, Reference Fagel and Hillaire-Marcel2006). Two cores – 646A (water depth of 3462 m) and 646B (water depth of 3459 m) – were combined into a composite section using magnetic and lithostratigraphic correlations (Srivastava et al., Reference Srivastava, Arthur, Clement, Aksu, Baldauf and Bohrmann1987). The upper 150 cm of sediments are only made of biogenic carbonates. At greater depth, the sediments are made of clayey and silty muds with a variable abundance of detrital carbonate-rich sandy layers, interpreted as turbiditic deposits. Oxygen isotope ratios measured from planktonic foraminifers were combined using biostratigraphy and magnetostratigraphy to define an age-depth model. The upper 32 m of core 646 were subsampled at 20 cm intervals (173 samples) for clay mineralogical analyses. This interval covered several glacial and interglacial periods, corresponding to the last 365 kyr (MISs 1–10). The sampling resolution was of 2500 years.

A second core (HU90-013-013; 58°12.59’N, 48°22.40’W) was collected on the south-west of Greenland Rise, north of the Eirik Ridge, at a water depth of 3380 m (Fig. 7). The site is bathed by DSOW overlain by NEADW (Lucotte & Hillaire-Marcel, Reference Lucotte and Hillaire-Marcel1994). The upper 530 cm of the core were composed of clayey silts and silty clays. Two sandy layers at 440–450 and 505–510 cm were interpreted as ice-rafted debris. Samples were retrieved between 240 and 530 cm, an interval covering the Last Glacial/Holocene transition (~8–26 kyr) according to the age model (Fagel et al., Reference Fagel, Hillaire-Marcel and Robert1997b). Sampling every centimetre provided a resolution between 10 and 160 years according to the sedimentation rate. The glacial interval (MIS2) was characterized by lower sedimentation rates (10 cm kyr^–1) than those of the Holocene interval (MIS1, > 30 cm kyr^–1; Hillaire-Marcel et al., Reference Hillaire-Marcel, de Vernal, Bilodeau and Wu1994).

A third core (HU91-045-094; 50°12.26’N, 45°41.14W) was retrieved from the southern Labrador Sea, in a deep channel on the Orphan Knoll at a water depth of 3448 m (Fig. 7). The bottom water mass is DSOW-like for core HU90-013-013. The 400 cm-long sediment core consisted of clays interlayered with abundant 10–40 cm-thick sandy layers. These layers displayed ice-rafted debris and a great detrital carbonate content, suggesting that ice surges on the Hudson Strait shelf (Andrews et al., Reference Andrews, MacLean, Kerwin, Manley, Jennings and Hall1995) triggered turbidites to flow down the North Atlantic Mid-Ocean Channel corresponding to these sediment layers (see location in Fig. 7; Hillaire-Marcel et al., Reference Hillaire-Marcel, de Vernal, Bilodeau and Wu1994). Such events were interpreted as the final collapse of the Laurentide ice sheet (Andrews et al., Reference Andrews, MacLean, Kerwin, Manley, Jennings and Hall1995). The core was subsampled at a lower resolution than core HU90-013-013 due to the presence of turbidite layers. A total of 68 samples were selected from 13 sediment intervals separating the turbidite layers.

For the three cores, the clay mineralogy was determined by XRD on the carbonate-free <2 μm fraction. Oriented mounts, prepared according to the glass slide method (Moore & Reynolds, Reference Moore and Reynolds1997), were analysed on a Siemens diffractometer with Co-Kα radiation. Semi-quantitative estimation of clay minerals was derived from the peak areas of smectite (17 Å), illite (10 Å) and chlorite + kaolinite (7 Å) on the traces from the glycolated XRD mounts, determined by multiplying peak heights by widths at mid-height (Fagel et al., Reference Fagel, Hillaire-Marcel and Robert1997b). The peak heights of kaolinite at 3.57 Å and chlorite at 3.54 Å were measured to estimate their relative proportions (Biscaye, Reference Biscaye1965). For the two cores located on the Greenland Rise (ODP646 and HU90-013-013), the clay mineral variation was expressed by the relative abundance of smectite and illite defined as their peak area ratio (S/I). In addition, the accumulation rates of smectite and illite (i.e. clay fluxes) were also evaluated by using Equation 1 (Fagel et al., Reference Fagel, Hillaire-Marcel and Robert1997b):

(1)$$\eqalign{{\rm Flu}{\rm x}_{{\rm clay\ mineral\ }( {{\rm g}/{\rm c}{\rm m}^2\;{\rm kyr}} ) } = & {\rm \delta} \times {\rm SAR} \times \% {\rm clay\ mineral\ } \cr & \times \% {\rm clay} {\hbox -}{\rm sized\ fraction}}$$

where δ is the mean clay mineral density (g cm^–3), SAR is the sediment accumulation rate (cm kyr^–1), % clay mineral is the abundance of the clay mineral in the fine <2 μm fraction determined by XRD and % clay-sized fraction is the proportion of the <2 μm fraction in the bulk sediment.

Site description

The Baikal Drilling Program (BDP) was the first initiative, begun in 1993, of long coring in continental settings, and it was developed with the aim to provide a continental archive ‘with the same scientific and chronostratigraphic integrity as marine records’ (Williams et al., Reference Kuzmin, Karabanov, Kawai and Williams2001). Located in south-central Siberia, Lake Baikal (Fig. 8) is the largest (636 km long × 79 km wide, 31.722 km²) and deepest (1642 m) lake in the world. It is located in an active tectonic rift zone, unaffected by continental ice sheets and out of direct oceanic influence (Hutchinson et al., Reference Hutchinson, Golmshtok, Zonenshain, Moore, Scholz and Klitgord1992). Its thick sedimentary cover (up to 5000 m) records tectonic and climatic changes over the last 20–40 Myr (Williams et al., Reference Williams, Peck, Karabanov, Prokopenko, Kravchinsky, King and Kuzmin1997). Lake Baikal sediments constitute powerful archives of past climate over Plio-Pleistocene glacial/interglacial cycles for south-central Siberia over the past 5 Ma (Williams et al., Reference Williams, Peck, Karabanov, Prokopenko, Kravchinsky, King and Kuzmin1997; Grachev et al., Reference Grachev, Vorobieva and Likoshway1998; Kuzmin et al., Reference Kuzmin, Karabanov, Kawai and Williams2001). Coring sites in the southern (BDP-93; BDP-93 End-Members, 1995) and northern basins (BDP-96, BDP-98; Fig. 8; Grachev et al., Reference Grachev, Vorobieva and Likoshway1998; Karabanov et al., Reference Karabanov, Prokopenko, Williams and Khursevich2000; Prokopenko et al., Reference Prokopenko, Karabanov, Williams and Khursevich2002) showed that Lake Baikal sediments were sensitive to climate fluctuations because there was greater diatom abundance during interglacial intervals and lower diatom abundance during glacial intervals. Lake Baikal provided an interesting opportunity to test the use of clay minerals for palaeoclimatic interpretation in a continental environment (e.g. Yuretich et al., Reference Yuretich, Melles, Sarata and Grobe1999; Sakai et al., Reference Sakai, Minoura, Soma, Tani, Tanaka and Nara2005). At present, Lake Baikal is fed by 90% river supplies, 8% atmospheric supplies and 2% groundwater discharges (Lomonosov et al., Reference Lomonosov, Khaustov, Gvozdkov and Shpeizer1995). Lake Baikal is mainly supplied by three rivers: the Selenga River (2172 kT yr^–1) in the southern sector of the lake, the Barguzin River (178 kT yr^–1) on the eastern side of the lake and the Upper Angara River (430 kT yr^–1) in the northern sector of the lake (Fig. 8), supplying 94% of annual particulate input. The types of source rocks in the Lake Baikal watershed were characterized and quantified using spatial analyses of Geographic Information System datasets (Fagel et al., Reference Fagel, Thamó-Bózsó and Heim2007). Palaeozoic granitoids dominate the watershed of the Selenga River (56%) with Cenozoic volcanic rocks (16%), mainly basalts, Tertiary and Quaternary sedimentary rocks (11%) and metamorphic Precambrian schists (7%). Archaean and Proterozoic intrusive rocks dominate in the Barguzin (50%) and Upper Angara (40%) watersheds. In terms of mineralogy, illite is the predominant mineral, derived from the micas of Proterozoic and Archaean granites outcropping on the south-east margin of the lake (Galasy, Reference Galasy1993). Chlorite, the second detrital clay mineral in the catchment of Lake Baikal, is mainly sourced from the Proterozoic metamorphic belt (schists, quartzites) along the western flank of the lake, south of the Siberian platform. The composition of secondary minerals varies according to the rate of weathering, which is controlled by the composition of parental rocks, topography and climate (Chamley, Reference Chamley1989). Therefore, the lacustrine sediments are expected to present a variable mineralogy through time.

Figure 8. Coring locations in Lake Baikal from various drilling campaigns: BDP (black circles; Yuretich et al., Reference Yuretich, Melles, Sarata and Grobe1999; Williams et al., Reference Kuzmin, Karabanov, Kawai and Williams2001); VER (abbreviation derived from the name of the Russian vessel RV Vereshagin; grey circles; Horiuchi et al., Reference Horiuchi, Minoura, Hoshino, Oda, Nakamura and Kawai2000; Fagel et al., Reference Fagel, Boski, Likhoshway and Oberhaensli2003); CON (Continent European proposal EVK2-CT-2000-00057; white circles; Fagel & Boës, Reference Fagel and Boës2008; Fagel & Mackay, Reference Fagel and Mackay2008).

The long-term evolution of clay mineral assemblages over a few hundred thousand years was investigated at a few coring stations on Buguldeika Saddle (Melles et al., Reference Melles, Grobe, Hubberten and Horie1995; Yuretich et al., Reference Yuretich, Melles, Sarata and Grobe1999) and Academician Ridge (Fagel et al., Reference Fagel, Boski, Likhoshway and Oberhaensli2003). Clay mineral data from BDP-93 cores from the Southern Central Basin (Yuretich et al., Reference Yuretich, Melles, Sarata and Grobe1999) showed that during the last 350 kyr there was a systematic increase of smectite in diatom-bearing sediments, indicating that a climate signature could be recorded in the clay-size fraction, which prompted further studies. Similarly to diatom productivity, the changes in smectite abundance were explained by temperature. Assuming all smectite is derived from soils, the observed trend by which smectite increased in warmer periods would reflect increased hydrolysis within the watershed during interglacials (Yuretich et al., Reference Yuretich, Melles, Sarata and Grobe1999). Subsequent studies are described here.

Materials and methods

Core VER98-1-3 was drilled on the northern part of Academician Ridge (53°44’56”N, 108°19’02”E) at a water depth of 373 m (Fig. 8). Academician Ridge is an intra-rift accommodation zone separating the central and north Baikal basins (Mats et al., Reference Mats, Khlystov, De Batist, Ceramicola, Lomonosova and Klimansky2000). The total length of the core was 1092 cm, but its lower part (835–1092 cm) was disturbed during the coring and therefore not investigated further (Fagel et al., Reference Fagel, Boski, Likhoshway and Oberhaensli2003). The sediment was made of clayey silty layers interbedded with several decimetre-thick massive or faintly laminated diatom-rich layers. The diatom-rich intervals were attributed to warm (interglacial) intervals characterized by enhanced biological productivity. The clay-rich intervals that are either massive, coarsely to finely laminated or bioturbated (Fagel et al., Reference Fagel, Boski, Likhoshway and Oberhaensli2003) were associated with colder (glacial) intervals. In core VER98-1-3, the abundance of diatoms followed a pattern similar to the standard marine oxygen isotope curve (SPECMAP) that records glacial/interglacial fluctuations. In sediments from Lake Baikal, the correlation between the diatom content and SPECMAP, the latter calibrated with radiocarbon dates for the latest Quaternary and Holocene, was indeed used as a stratigraphic tool (Grachev et al., Reference Grachev, Vorobieva and Likoshway1998) to extrapolate further back in time (before 35 kyr). In this study, the age model was based on the correlation between the depths at which diatom assemblages VII–XV were found in reference sections from Academician Ridge (Likhoshway, Reference Likhoshway, Manami, Idei and Koizumi1998) and in core VER98-1-3. Assuming a constant sediment accumulation rate, the biostratigraphic correlation suggested that the last 54 kyr, corresponding to the uppermost 2.8 m of the sediments, are missing. The upper 8 m of core VER 98-1-3 probably recovered a time interval from 55 to 250 kyr, covering four interglacial/glacial intervals, labelled oxygen isotope stages (OISs) 4 to 8.

Core CON01-603-2a (53°96’N, 108°91’E, water depth of 386 m) was drilled in the northern basin of Lake Baikal on an extension of Academician Ridge called Continent Ridge (Fagel & Boës, Reference Fagel, Hillaire-Marcel and de Vernal2007). The sediment from bottom to top was composed of silty clays with a few layers of diatoms (128–141 cm, unit 3), finely to coarsely laminated clayey silts to silty clays with abundant diatoms (9.5–128 cm, unit 2) and diffuse to finely laminated diatom-rich mud (0–9.5 cm, unit 1). The age model was derived from magnetic susceptibility and anhysteresis remanent magnetization measurements (Demory et al., Reference Demory, Nowaczyk, Witt and Oberhänsli2005) by correlation with the ODP 984 reference site (Noth Atlantic; Channell, Reference Channell1999) using seven correlation points between 10.5 and 141.8 cm (Fagel & Boës, Reference Fagel and Boës2008). In core CON01-603-2a, the sedimentation rate averaged 10 cm kyr^–1 in the middle unit (unit 2). The lowest sedimentation rates (~3.8 cm kyr^–1) were observed in unit 3 and at between 66 and 89 cm in unit 2. The entire core covered the last 22 kyr, with the Late Glacial/Holocene transition happening at 76 cm. The age of the surface sediments was estimated at 2.8 kyr BP by extrapolation of the youngest measured sedimentation rate.

Core CON01-604-2a (52.08°N, 105.86°E) was retrieved on Posolsky Bank (Fig. 8) in the vicinity of the Selenga River delta at a water depth of 133 m (Fagel & Boes, Reference Fagel, Hillaire-Marcel and de Vernal2007). Posolsky Bank is a tilted fault block within the Selenga Delta Accommodation Zone (Hutchinson et al., Reference Hutchinson, Golmshtok, Zonenshain, Moore, Scholz and Klitgord1992). The sediments consisted of homogeneous silty clays at the bottom (105–110 cm, unit 3), coarsely to finely laminated silt (25–105 cm, unit 2) and homogeneous mud at the top unit (0–25 cm, unit 1). The palaeomagnetically derived age models for core CON01-604-2a (Demory et al., Reference Demory, Nowaczyk, Witt and Oberhänsli2005) were constrained by seven correlation points between 9 and 130 cm with the ODP984 reference site (Channell, Reference Channell1999). In core CON01-604-2a, the sedimentation rates ranged between 6.5 and 9.2 cm kyr^–1 in unit 2 and increased in both the upper and lower lithological units (11 cm kyr^–1 in unit 1, 16 cm kyr^–1 in unit 3). The entire core represented an interval ranging between 2.4 and 14 kyr, with the Late Glacial/Holocene transition occurring at 75–76 cm (Fagel & Boës, Reference Fagel and Boës2008).

The mineralogy of core VER98-1-3 was studied with XRD at low resolution (1 sample per 10 cm), whereas XRD analyses were performed at every centimetre in cores CON01-603-2a and CON01-604-2a, corresponding to an average temporal resolution of ~100 years. Qualitative and semi-quantitative estimations of clay mineral assemblages were based on XRD peak intensity measurements made on oriented, glycolated aggregates (Fagel et al., Reference Fagel, Boski, Likhoshway and Oberhaensli2003), and these analyses were also accompanied by those of air-dried and heated samples for comparison. XRD analysis was carried out on a Philips PW 1390 diffractometer using Cu-Kα radiation. Smectite presence was indicated in two ways. First, illite–smectite was indicated by a shoulder of the low-angle side of a peak at 14 Å (Thorez, Reference Thorez and Lelotte1976). The abundance of this illite–smectite phase was estimated by the difference between the intensity of the 10 Å peak before and after heating (Boski et al., Reference Boski, Pessoa, Pedro, Thorez, Dias and Hall1998). Second, a peak was observed at ~17 Å on both the air-dried and glycolated samples. This behaviour is common in soils and was interpreted as being due to the presence of Al-hydroxides within the interlayers, called here Al-smectite (Thorez, Reference Thorez2000). The 10 Å peak indicated illite, while those at ~14 Å and ~7 Å indicated chlorite and kaolinite.

For clay mineral quantification, the intensity of the 10 Å peak was used as a reference and the peaks of the other minerals were divided by a weight factor (2.5 for 10–14 Å and chlorites, 1.4 for kaolinite, 5 for Al-smectite) and all identified clay species were normalized to 100%. These corrective factors were determined empirically at the University of Liège (J. Thorez, pers. comm. 2001). An S/I ratio was estimated on the glycolated diffractograms, reflected the abundance of illite–smectite and Al-smectite relative to illite. Assuming all smectites formed by transformation processes, the S/I ratio may be used as a proxy for hydrolysing conditions in the lake watershed (Fagel & Boës, Reference Fagel and Boës2008).

In addition, Li saturation was performed on samples from core VER98-1-3 to investigate the composition of smectite. The abundances of montmorillonite and beidellite were estimated by comparing the intensities of the 10 Å peak on the XRD runs of three Li-saturated samples (Thorez, Reference Thorez1998) as follows: montmorillonite = I(Li-300GL) – I(Li_N); beidellite = I(Li-300) – I(Li-300GL), where GL is glycerol solvation. The relative contribution of Al-smectite was based on the intensity of the 17 Å peak in the Li-300 trace, which was then reported on the glycolated trace (Fagel & Boës, Reference Fagel, Hillaire-Marcel and de Vernal2007).

Site description

Since ~1970 CE, the East African Rift has been the subject of many palaeoanthropological and palaeoclimate studies searching for causal relationships between hominin evolution and climate changes (see Campisano et al., Reference Campisano, Cohen, Arrowsmith and Asrat1997 for a review). Ethiopia in particular is characterized by an impressive accumulation of Pliocene fossil hominins (e.g. Johanson et al., Reference Johanson, Taieb and Coppens1982) and late Pliocene flora and mammalian fauna (e.g. Bonnefille et al., Reference Bonnefille, Potts, Chalie, Jolly and Peyron2004; Reed, Reference Reed2008).

Within this framework, an ICDP project (the Hominin Sites and Paleolakes Drilling Project; HSPDP) was launched to collect palaeoenvironmental data from lacustrine sediments close to key palaeoanthropological sites in Kenya and Ethiopia (Cohen et al., Reference Cohen, Arrowsmith, Behrensmeyer, Campisano, Feibel and Fisseha2009, Reference Cohen, Campisano, Arrowsmith and Asrat2016; Campisano et al., Reference Campisano, Cohen, Arrowsmith, Asrat, Behrensmeyer and Brown2017, Foerstner et al., Reference Foerster, Asrat, Bronk Ramsey, Brown, Chapot and Deion2022). Among others, the Chew Bahir basin in south-west Ethiopia was analysed by Foerster et al. (Reference Foerster, Junginger, Langkamp, Gebru, Asrat and Umer2012) to reconstruct past environmental conditions and improve our understanding of the relationship between climate, environment and human dispersion in Africa. The Chew Bahir basin today is a dried-out saline mudflat laying in a transition zone between the Ethiopian Rift and the Omo-Turkana basin. This palaeolake was a closed basin (without any water outlet) receiving the totality of the weathering products from its catchment (Foertsner et al., Reference Foerster, Vogelsang, Junginger, Asrat, Lamb, Schaebitz and Trauth2015). The western margin of the Chew Bahir basin is the Hammar range, which consists of Precambrian gneissic rocks. On the eastern margin, the Teltelee-Konso range contains Miocene basalts and trachytes. Oligocene basalts outcrop at the northern margin of the lake. The basin was infilled by ~5 km of sediments since the Miocene rifting. The fluviatile supplies are limited to the northern part of the basin, whereas the western and eastern margins are drained by rain. No sediments are supplied from the south lake margin. Sediment influx is seasonal, related to rainfall events during the wet season and to wind during the dry season, when it may become dominant.

Materials and methods

The core CB-01-2009 was recovered from the western margin of the Chew Bahir basin (04°50.6’N, 36°46.8’E) near an alluvial fan extending eastwards from the Hammar range (Foerster et al., Reference Foerster, Junginger, Langkamp, Gebru, Asrat and Umer2012). The sediment core covers the uppermost 18.86 m of the deposits, corresponding to ~45 kyr according to six radiocarbon age measurements (mean sedimentation rate 0.7 mm yr^–1). The lithology consists of silty clays intercalated with sandy layers and gravels. CB-01 records climate history spanning from 44 to 1.0 kyr, which includes the Last Glacial Maximum (18–23 kyr) and the African Humid Period (5–15 kyr) and ends at the onset of the Medieval Warm Period (700–1000 BP/950–1250 CE). The mineral content was determined by XRD analysis of bulk samples, using a Siemens D5000 diffractometer and the software EVA (Bruker) for qualitative phase identification. Elementary composition of the sediment core was determined by X-ray fluorescence with an Itrax core scanner (CS-XRF) equipped with a molybdenum tube. The core was scanned at 0.5 cm resolution with a voltage of 30 kV and a current intensity of 30 mA, with a scanning time per step of 20 s. The clay fraction was separated according to the protocol of Moore & Reynolds (Reference Moore and Reynolds1997) and analysed on an EMPYREAN PANalytical XRD device using Cu-Kα radiation. Three diffractograms were measured on oriented mounts for each sample (air-dried, ethylene glycol-solvated and heated at 550°C for 2 h). The phase identification was performed using HighScore Plus version 4.0 software.

Site description

The East African Rift valley is an active tectonic area with a particularly intense phase of rifting from the Cretaceous to the Miocene, leading to the formation of numerous graben lakes (Tiercelin & Lezzar, Reference Tiercelin, Lezzar, Odada and Olgado2002; Pickford et al., Reference Pickford, Senut and Cheboi2009) filled with lacustrine and fluviatile sedimentary deposits and later with Middle Miocene to recent volcanic rocks. Fault reactivation in the Tugen Hill area was responsible for the formation of the Lukeino depression in which the palaeolake Lukeino developed (Pickford, Reference Pickford1974). The base of the formation is made by the Kabernet trachyte of 6.09 ± 0.14 Myr. The Lukeino Formation is sealed by the Kaparaina basalts (5.68 ± 0.18 Myr) that filled the depression and obliterated the palaeolake (Pickford, Reference Pickford and Bishop1978; Sawada et al., Reference Sawada, Pickford, Senut, Itaya, Hyodo and Miura2002).

The Lukeino Formation outcrops along the central part of the East African Rift, east of the Tugen Hills in Kenya, over an area 44 km wide × 13 km long (Bamford et al., Reference Bamford, Brigitte Senut and Pickford2013). Sawada et al. (Reference Sawada, Pickford, Senut, Itaya, Hyodo and Miura2002) proposed a simplified lithological column of the Lukeino Formation, divided into three members from bottom to top: (1) the Kapgoywa Member between the Kabernet trachyte and the Kapsomin basalt; (2) the Kapsomin Member; and (3) the Kapcheberek Member located between the Rormuch Sill and the Kaparaina basalt (Fig. 12). The Lukeino Formation was interpreted as a fluvio-lacustrine sedimentary sequence made by clayey and sandy clayey deposits intercalated by the Kapsomin basalts and the injection of the Rormuch Sill (Fig. 12; Pickford, Reference Pickford1975, Reference Pickford and Bishop1978). This formation has yielded numerous floral, faunal and hominid remains (Bamford et al., Reference Bamford, Brigitte Senut and Pickford2013) and it is known for revealing the oldest East African bipedal hominid, called Orrorin tugenensis (Senut et al., Reference Senut, Pickford, Gommery, Mein, Cheboi and Coppens2001). This formation was investigated in detail to better constrain the environment in which the hominids lived (e.g. Pickford et al., Reference Pickford, Senut and Cheboi2009; Bamford et al., Reference Bamford, Brigitte Senut and Pickford2013).

Figure 12. (Left) Evolution of the mineralogical assemblages of the Lukeino Formation derived from bulk XRD. The stratigraphic positions of the three Kapgoywa, Kapsomin and Kapcheberek members were based on lithological observation, magnetic susceptibility measurements and positions of the volcanic rocks. (Right) Palaeoenvironmental interpretation of the Lukeino sedimentary deposits derived from the peak area ratio between smectite and kaolinite (modified from Dericquebourg, Reference Dericquebourg2016). The units correspond to sections 1, 2 and 3 on the left. K = kaolinite; S = smectite.

Materials and methods

Sedimentary samples were collected in 2004, 2010 and 2011 within the framework of the ‘Kenya Paleontological expedition’ lead by B. Senut and M. Pickford from the National Natural History Museum of Paris in France. In total, 14 geological sections from six sites (Aragai, Cheboit, Kapgoywa, Kapsomin, Kapcheberek and Sunbarua) were investigated to cover the whole sequence of the Lukeino Formation (Dericquebourg, Reference Dericquebourg2016). The sampling interval ranged between 0.1 and 1.0 m. The bulk mineralogy was studied on 480 horizons using a Bruker AXS D2-phaser equipped with Cu-Kα radiation and a rapid LynxEye detector (ISTeP, Université Pierre et Marie Curie, Paris). The analyses were carried out between 2 and 75°2θ, with a step of 0.02°2θ and a counting time of 0.1 s per step, using a voltage of 30 kV and a current intensity of 10 mA. XRD traces were analysed using MacDiff software (Petschick, Reference Petschick1997). The semi-quantitative abundance of the minerals was derived from the measurement of peak surface, with an uncertainty of ±5% (Moore & Reynolds, Reference Moore and Reynolds1997). Concerning the clay minerals, a kaolinite/smectite ratio was calculated from the surface ratios of the 001 peaks of kaolinite (7 Å) and smectite (15 Å) observed on the bulk XRD trace. This ratio was used to identify well-drained periods characterized by greater abundance of kaolinite over smectite (K/S > 1) and dry periods marked by the greater presence of smectite (K/S < 1), following a mineralogical study of Late Miocene fluvio-lacustrine deposits on the Ethiopian Plateau (Yemane et al., Reference Yemane, Robert and Bonnefile1987). In this study, the formation of both kaolinite and smectite was related to pedogenetic processes, as their distribution was not correlated with the presence of volcanic materials. In soils, kaolinite is abundant in high-relief areas and/or in warm and humid environments formed under efficient drainage increasing the removal of cations and a portion of the dissolved silica (Millot, Reference Millot1970; Chamley, Reference Chamley1989). Smectite is generally abundant in low-relief areas, where poor drainage prevents the removal of cations and silica. Its formation in soils is favoured by alternating wet and dry periods (Millot, Reference Millot1970; Chamley, Reference Chamley1989). Therefore, for the Ethiopian sedimentary sequence, Yemane et al. (Reference Yemane, Robert and Bonnefile1987) emphasized that intense rainfall favoured the formation and the subsequent erosion of kaolinitic soils, whereas smectite-rich soils are instead developed during drier periods. The same interpretation was applied for the Lukeino Formation, as presented below.