Taxonomy and palaeoecology of Ostracoda from the Middle to Late Pleistocene upper Karewa formation of Kashmir Valley, Northern India

Ostracoda from a 25 m thick exposure in sediments of the upper Karewa formation indicate lacustrine conditions in the Kashmir intermontane basin during the Middle to Late Pleistocene. The Middle to Late Pleistocene age is established on the basis of lithostratigraphy and confirmed by two thermoluminescence age determinations at the top of the section. The ostracod assemblages show that the lake water remained fresh or, at most, was very slightly saline, throughout the period represented by the section. Nearly all ostracod taxa identified from the sediments are cold stenothermal forms and they include inhabitants of the littoral and profundal domain as well as running waters and springs. The Ostracoda present within the section indicate that during deposition of the upper Karewa sediments a large, but fairly shallow, well-oxygenated open-basin freshwater lake existed in that central part of the Kashmir Basin. Although the sequences suggest changes in water depth may have occurred, lacustrine conditions were otherwise largely unchanged over the period represented by the section, up until the demise of the lake during the early part of the Late Pleistocene.


INTRODUCTION
opinions on the formal stratigraphic nomenclature of the sediments vary and in this paper we follow Burbank & Johnson (1983) in distinguishing informally between upper and lower Karewa sediments. The lower Karewa, which is the thicker of the two units (up to about 1300 m: Burbank & Johnson, 1983), consists of lacustrine, deltaic and fluvial facies with four conglomerate units. During the lacustrine phases, the lake occupied much of the basin floor. The upper Karewa, which is the thinner of the two units (%100 m), is separated from the lower Karewa by an angular unconformity in the basin centre and a conglomerate bed along the Pir Panjal margin of the basin. The conglomerate bed marks rapid uplift of the Pir Panjal Range around 0.4 Ma BP (Burbank & Johnson, 1983). This uplift led to drainage of the large lower Karewa Lake and tilting of the lower Karewa sediments leading to downcutting and subsequent exposure, especially along the Pir Panjal margin of the basin. The upper Karewa sediments comprise, from the base of the unit upwards, the final conglomerate bed, lacustrine sediments of a restricted upper Karewa Lake and finally loess. Following the uplift of the Pir Panjal Range, the Karewa Lake became shallower and smaller in area as a result of drainage. Loess deposition took place on the exposed conglomerate surfaces along the south-western basin margin and also into the lake itself: the upper Karewa Lake sediments are silt-rich and some of this silt was undoubtedly of aeolian origin. The lake finally drained in the Middle to Late Pleistocene (Holmes et al., 1992), giving way to loess deposition across the entire basin floor (Rendell & Townsend, 1988). Subsequent downcutting has exposed the upper Karewa sediments in river valleys and gullies throughout the basin. Several residual lakes, notably Dal Lake, Manasbal Lake and Wular Lake, remain in the basin today, but they are tiny remnants of the larger lake that existed in the past. Bhatt (1989) proposed a simple model showing the evolution of the Karewa Lake from basin inception, through rapid uplift of the Pir Panjal around 0.4 Ma BP and up to the present day (Fig. 3). Lake drainage is assumed to have been through the Baramulla Gorge (Fig. 4), the pathway taken by present-day drainage from the basin.

PREVIOUS WORK
There are numerous publications on ostracods from the Karewa deposits and from the modern residual lakes in the basin.  provided a description of ostracod taxa found at a number of localities in the upper Karewa sediments. However, there is no stratigraphical information given about the samples and his palaeoenvironmental conclusions are highly generalized. None the less, he was able to conclude that the upper Karewa Lake was large, permanent and slightly alkaline.  suggested that it was fed by a series of sluggish, vegetation-rich streams and was similar in character, if not in size, to the rather smaller present-day Wular Lake in Kashmir. Other, mainly taxonomic, descriptions of ostracods from the upper Karewa Lake include those of Singh (1972Singh ( , 1977: some of the taxa reported in his papers suggest the existence of small marginal ponds around the edge of the upper Karewa Lake. Singh (1973) argued that the deepest part of the lake extended through the centre of the basin and included the site at Pattan that forms the basis of the present paper. Ostracods are also described from the lower Karewa deposits (e.g. Singh, 1972Singh, , 1973Singh, , 1977 and indicate that this earlier, larger lake was deeper than the upper Karewa Lake and also organic rich, a conclusion that is supported by the relatively high organic content of much of the lower Karewa strata. Limited descriptions of modern ostracods from Kashmir can be found in Bhatia & Singh (1971) and Singh (1972).
Despite the wealth of previous work on ostracods from Kashmir, many of the collections are not accompanied by detailed stratigraphical diagrams or information about sampling localities. As Bhatt (1989) has pointed out, the results of these investigations can be used only in a general way to inform on the palaeonvironment during upper Karewa times. Moreover, the taxonomy of some of the species described is open to question and requires reassessment in light of recent developments in the study of non-marine ostracods (e.g. Meisch, 2000). In this paper, we describe the ostracod fauna from a logged and dated section through the upper Karewa sediments at a site near Pattan. This section is especially important, because it preserves the transition from lacustrine to aeolian sedimentation and thus records the demise of the upper Karewa lake in the central part of the basin. We describe the occurrence of ostracod species from the 25 m section and discuss their taxonomy and palaeoecological significance.  Middlemiss (1910), Wadia (1934) and Burbank & Johnson (1983).

MATERIAL AND METHODS
Ostracoda from the upper Karewa lacustrine sediments were studied from strata exposed in a gully near the village of Pattan (34.1456(N: 74.5462(E; 1676 m above sea-level (a.s.l.)), c. 25 km WNW of Srinagar (Fig. 4). Here, around 8-10 m thickness of loess overlies conformably the upper Karewa lake sediments. A total of 29 m of loess and lake sediments was exposed by digging vertical faces into the gully side. The gully extends a further 10-15 m below the base of the measured section, but the accumulation of slumped and washed material at the base made further excavation impossible. From the stratigraphy of the upper Karewa sediments elsewhere in Kashmir, the upper Karewa conglomerate is assumed to underlie the lake sediments at this site. However, despite extensive searches in this and neighbouring gullies, no trace of conglomerate beds was found and the unit was assumed not to be exposed in this locality. Samples for ostracod analyses were collected from the exposed faces in the gully using a trowel and dustpan. In all, 82 samples were taken for the analysis of ostracod faunas. Individual samples covered about 5 cm stratigraphical thickness and were generally taken at 25 cm intervals throughout the section, although minor adjustments were made to sampling intervals or sample thickness to avoid sampling across observed boundaries in sediment units.
In the laboratory, 200 g of each sample was dispersed in deionized water and washed through a 63 µm sieve. The residue was dried at 30(C and first checked under a binocular microscope for ostracod abundance within the sample. Because of wide variations in abundance, many of the samples were split to varying degrees using a mechanical splitter. Appropriate splitting factors were used to correct abundances after counting of species from the slides. In order to facilitate picking from the slides, the dry residue was again sieved through 355 µm, 250 µm and 180 µm sieves, after which the material was amalgamated prior to taxonomic sorting.
Scanning electron micrographs were produced on a Cambridge Instruments Stereoscan 360 electron scanning microscope. For preparation, ostracods were attached on a carrier with glue strips and then gold-coated for 6 minutes at vacuum pressure.

Stratigraphy of the section
The section at Pattan is composed predominantly of laminated to massive fine-grained sediments, mainly inorganic silty clays or silty-fine sands, with metre-scale medium to coarse sand units (Fig. 5). The finer sediments are typically calcareous (CaCO 3 from 3-12%) and poorly to very poorly sorted, with mean grain size varying from about 2 µm to 10 µm. The sand units are planar cross-bedded, slightly calcareous to non-calcareous (CaCO 3 <1%), and poorly to very poorly sorted with mean grain size from 250 µm to 600 µm. The fine-grained silts become decalcified in the upper c. 5 cm of the lake-sediment unit close to the boundary with the loess, probably as a result of pedogenic activity.

Ostracoda
Ostracoda are an essential part of most of the sampled horizons from the section near Pattan. Only a few intervals lack ostracods; these are mainly the medium to coarse sands and the decalcified sediments close to the lake-sediment to loess transition (Fig. 5). Abundances of 200 to 1000 adult valves per 200 g sediment were commonly observed; some samples contained more than 4000 valves. The assemblages include several juvenile stages in most cases, exceptions occur only for rare taxa, such as Prionocypris zenkeri and Potamocypris zschokkei. In all, 15 taxa out of three superfamilies were identified from the Pattan section. Identification and taxonomy follow Hartmann (1964), Singh (1977), Meisch (2000) and Bates et al. (2002). Selected taxa are illustrated in Plates 1 and 2.

Pleistocene Ostracoda of Kashmir
Ecology. This is found in a variety of habitats, although it occurs chiefly in permanent lakes (Singh, 1977). It is a nonswimmer and burrows in the upper surface of lake floors, particularly amongst vegetation detritus (Robinson, 1980). From laboratory experiments, it is known that D. stevensoni can tolerate salinities ranging from distilled water to almost full sea water, but under natural conditions it reaches maximum densities in waters of up to 2 g l 1 total dissolved solids (TDS) (van Doninck et al., 2003a). The Darwinuloidea represent an almost exclusively asexual lineage over the last 200 million years, although rare living males have been found (Smith et al., 2006). The life cycle of D. stevensoni depends on the seasonal temperatures and is prolonged (up to four years) in northerly latitudes close to the Arctic Circle (Ranta, 1979). In a study undertaken in Belgium, for example, the reproductive period started during March when water temperatures exceeded 10(C, leading to maximum population densities during June/July (Van Doninck et al., 2003b). Adults tolerate temperatures between 10(C and 30(C (Van Doninck et al., 2003a).
Geographical distribution. Alpine and prealpine lakes of Europe, North America, Russia, Asia.

Ecology.
A typical cold-water species preferring freshwater, but can tolerate slightly saline water (maximum salinity: c. 6‰) (Hiller, 1972;Meisch, 2000). It is reported from nearly all types of aquatic habitat. The eggs of Candona candida are desiccation resistant, which allows the species to survive in temporary waterbodies.
Geographical distribution. Europe, North America, Asia (Holarctic). C. neglecta as an indicator of the sublittoral zone. It prefers cold waters but tolerates a temporary increase in temperature above 20(C and a fairly wide range of salinities from 0.5‰ to 16‰ (Meisch, 2000). Like most other members of the genus, it is a burrowing form and is often found amongst lakebottom detritus (Preece et al., 1986). Juveniles of C. neglecta and other candonids have sometimes been mistaken for a separate species under the name Candona lactea (Preece et al., 1986).
Geographical distribution. Europe, North America, Middle East, Siberia.
Subfamily Cyclocypridinae Kaufmann, 1900 Genus Cypria Zenker, 1854 Cypria ophtalmica (Jurine, 1820) (Pl. 1, fig. 15) Ecology. Occurs in a wide range of aquatic habitats, including lakes, streams, ponds, springs, cave waters and bogs (Meisch, 2000). In lakes it has been found in littoral environments as well as down to the great depths of the profundal zone (Meisch, 2000). It is also a good swimmer (Bhatia & Singh, 1971). Cypria ophtalmica has a temperature optimum between 2.2(C and 8.4(C and probably belongs to the cold stenothermal forms (Mallwitz, 1984). Reported salinity tolerance ranges from freshwater to lower oligohaline (%5.8‰), but it has also been collected from waters with salinity of c. 25‰ (polyhaline) as inferred from chloride concentration (Meisch, 2000). It produces two generations annually and eggs that are desiccation resistant.

Genus Cyclocypris Brady & Norman, 1889
Cyclocypris ovum (Jurine, 1820) (Pl. 1, figs 18-19) Ecology. A common species that inhabits a wide range of aquatic habitats. It is especially common in the littoral zone of lakes but can be found in deeper waters, occasionally as deep as c. 70 m (Meisch, 2000). In the Qilian Mountains of China it was abundant in the surface sediment of a freshwater (salinity of 0.5‰) lake at an altitude of 3200 m, but was rare in running waters (Mischke et al., 2003). Maximum salinity in which the species has been reported is 6.4‰ (Meisch, 2000). It is generally associated with permanent waters, although it has been reported from temporary habitats (Meisch, 2000).

Stratigraphical range. Miocene-Recent.
Remarks. This species may be identified conclusively by presence of a number of riplets on the inner margin of the left valve (Janz, 1994). However, these riplets are not always well preserved and, indeed, were not observed on the material from the Pattan section. We, therefore, refer to the material from Pattan as Ilyocypris cf. bradyi.
Ilyocypris salebrosa Stepanaitys, 1960 (Pl. 2, figs 13-15) Ecology. Living specimens of this species are known from the shallow freshwater Dal Lake in Kashmir valley, where its presence is attributed to a small sluggish tributary flowing into the lake from the north (Singh, 1977). Its wider presence within Pleistocene deposits of Europe, North America and China is in contrast to the very restricted populations today (Mischke & Schudack, 2001;Bates et al., 2002).

Ecology.
A cold stenothermal form living in slow-flowing and standing waters, springs and shallow ponds (Hartmann, 1964). The presence of reduced natatory setae (antennule A2) in living specimens points to an epibenthic crawling or burrowing locomotion (Hartmann, 1964). Eucypris afghanistanensis is found in Quaternary lacustrine sediments of the Himalaya up to altitudes Pleistocene Ostracoda of Kashmir of 4500 m (Hartmann, 1975), where it is commonly associated with candonid ostracods as well as molluscs and Chara. In the Tso Kar Basin (Ladakh, N. India, 4500 m a.s.l.), recent valves of E. afghanistanensis were collected from the littoral zone of a lake and from shallow temporary ponds, with electrical conductivities ranging from 180 to 900 µS cm 1 (corresponding to a salinity range of c. 0.1‰ to 0.7‰) and surface water temperatures of 18(C to 25(C (M. Kramer,unpublished). It has also been reported (as Parastenocypris sp.) from Quaternary deposits in the Lamayuru Basin at 3600 m a.s.l. in Western Ladakh (Kotlia et al., 1997). Eucypris afghanistanensis was first described by Hartmann (1964) from the Middle East. During ontogeny a variety of valve shapes is realized, whereby adult specimens range in length from 1.20 mm to 1.40 mm and have a height/length ratio of 0.44 to 0.49. On the basis of valve shape description, it is synonymous with Parastenocypris delormi Singh, 1974, reported from Kashmir and possibly Eucypris gyirongensis Huang, 1982 from the Tibetan Plateau. Potamocypris zschokkei (Kaufmann, 1900) (Pl. 1, figs 13-14) Ecology. A cold stenothermal species (temperature optimum of the parthenogenetic form c. 9(C), which shows a preference for shallow, slowly running waters (Meisch, 1984). It also occurs in springs and interstitial habitats. It produces two to three generations annually and is associated with permanent waters (Meisch, 1984(Meisch, , 2000.
Subfamily Cypridopsinae Kaufmann, 1900Genus Cypridopsis Brady, 1867Cypridopsis vidua (O. F. Müller, 1776 (Pl. 1, fig. 4) Ecology. This common and prevalent ostracod species is found in a wide range of habitats, including lakes, rivers, ponds, springs, wells, interstitial habitats and rice fields (Meisch, 2000;Rossi et al., 2003). In lakes it prefers the vegetated littoral and sublittoral zones (Carbonel et al., 1988). Generally, the occurrence of C. vidua appears to be independent of current conditions (Curry, 1999). It is an active swimmer that can tolerate total dissolved solids up to 10.5 g l 1 (Curry, 1999), but is intolerant of low dissolved oxygen saturation (Meisch, 2000). This summer form produces two generations annually, the first in early spring and the second in late summer, whereby only few adults overwinter (Meisch, 2000). The species is exclusively parthenogenetic.

DISCUSSION
The sediments at Pattan are predominantly lacustrine silts with minor units of fluviodeltaic sand. The existence of ripple laminae suggests a shallow waterbody: based on investigations at Pattan and elsewhere within the basin (Singh, 1982), the facies suggest that the water was often no more than about 2 m deep, only exceptionally deeper. The presence of large quantities of silt within the sediments suggests a high degree of aeolian input. The top of the Pattan section was dated to c. 118 ka by means of thermoluminescence (Holmes et al., 1992). Although the base of the measured section is undated, it must be younger than 0.4 Ma BP, which is the estimated age of the upper Karewa conglomerate unit that is assumed to underlie the lake sediments. Luminescence dates from elsewhere in the basin indicate that the demise of the upper Karewa Lake probably took place sometime between about 80 kaBP and 110 000 kaBP (Rendell & Townsend, 1988;Gardner & Rendell, 1994).
In general, the ostracod species from the Pattan section show broad environmental tolerances and habitat preferences. Seven of the fifteen taxa found can tolerate freshwater conditions (salinity %0.5‰), but nearly all share a preference for oligohaline conditions (salinity 0.5-5‰). However, few of the taxa can tolerate waters with salinities greater than 15‰.
Ostracods such as Limnocytherina sanctipatricii, Candona neglecta, Darwinula stevensoni or Cytherissa lacustris, which are commonly found inhabiting the profundal zones of lakes, are abundant in the Pattan sediments. Similar findings are reported by Singh (1973), who underlines the profundal character of the ostracod fauna documented near Pattan. This supports Bhatt's (1989) suggestion that the site falls within the deepest central part of the upper Karewa Lake. Furthermore C. lacustris and L. sanctipatricii are indicative of well-oxygenated bottom water conditions, which are typically present in cold and oligotrophic lakes (Danielopol et al., 1985;Carbonel et al., 1988;Scharf, 1991;Löffler, 1997). Oxygen-saturated bottom waters may reflect low biological productivity within the lake, permanent circulation or inflow of cold and oxygen-saturated waters from glacial meltwater. However, low biological productivity is not consistent with the presence of taxa that show affinity for aquatic plants. Moreover, the contribution of glacial meltwater to the upper Karewa Lake is a matter for debate. Small glaciers are present in both the Pir Panjal and Great Himalayan Mountain ranges at present, although the total glaciated area is small. Glacial advances during the Pleistocene would undoubtedly have increased the extent of glacial ice within the mountain ranges. However, the magnitude of those advances was limited and their timing uncertain (Holmes & Street-Perrott, 1989). Moreover, the timing of deposition of the sediments at Pattan is not well constrained, and it remains unclear whether, climatically, they represent a cold (glacial) or warm (interglacial) stage.
The ostracods Ilyocypris salebrosa, Potamocypris zschokkei and Prionocypris zenkeri are typically reported from running waters and may be linked to the influence of inflowing rivers and streams, despite the fact that the Pattan section lay towards the basin centre. Significantly, however, these taxa show low abundances and lack complete ontogenies, which would tend to confirm that they are not in situ but have been transported by current action. Typical representatives of the littoral or the transitional zone towards running water are also common in the Pattan fossil assemblages. These are E. afghanistanensis, Pseudocandona cf. compressa and Ilyocypris cf. bradyi.
The distribution of ostracod taxa within the Pattan section shows some stratigraphical variation (Fig. 5). The most notable changes are the reduction in the proportion of E. afghanistanensis and the rise in D. stevensoni and C. ovum in the upper half of the sequence. The association of E. afghanistanensis with running water is consistent with the occurrence of several prominent sand units in the lower 12 m of the section. It is possible that the lake deepened during the latter part of the period represented by the section, such that the influence of inflowing streams was reduced as the distance between the Pattan site and the lakeshore increased. However, there is no other clear evidence within the ostracod assemblages of a significant increase in water depth and, for the most part, the ostracods suggest that the lake was cold and well oxygenated at this site throughout the time represented by the section, with abundant aquatic plants.
In summary, the Ostracoda from Pattan show affinities to profundal, littoral and running water habitats. The lake water can be characterized as freshwater to slightly saline, ranging from 0.5‰ to 5‰ on the basis of ostracod assemblages. The trace-element analyses undertaken on ostracods from this section (Holmes et al., 1989) also point to a lake that was, at most, only slightly saline. Nearly all taxa are cold stenothermal forms, which may reflect the influence of surface water supply from nearby glaciers or generally cool air temperatures. The sediments of the upper Karewa formation mainly represent lacustrine conditions over large parts of the Kashmir Valley, with a deeper part stretching along tectonic strike of the basin (Singh, 1973). The ecological aspects of the ostracod assemblages from the section near Pattan are consistent with this suggestion. Although there is some ostracod evidence to suggest that the lake deepened during the latter part of the time represented by the Pattan section, limnological conditions remained broadly unchanged. The final drainage of the lake, in the early part of the Late Pleistocene, appears to have been quite abrupt, with no evidence for gradual shallowing. This is consistent with the previously published ostracod trace-element data for this sequence. Once the lake had drained, lacustrine sedimentation was replaced by the accumulation of aeolian silt.