The ostracod record from Kajemarum Oasis in the Sahel zone of Northeastern
Nigeria covers the last c. 4 000 cal, years of a 5 500 cal. year lake-sedim
ent sequence. The first appearance of ostracods, around 4 000 cal. yr BP, r
eflects the switch from a very dilute lake during the mid-Holocene, to slig
htly oligosaline conditions that favoured the occurrence and preservation o
f ostracods. Between 3 800 and 3 100 cal. yr BP, the lake remained permanen
t and fresh or slightly oligosaline, with a Ca-Mg-HCO3 composition. A rise
in salinity c. 3 100 cal. yr BP, accompanied by a change to more variable c
onditions on a seasonal to interannual timescale, led to the influx of more
-euryhaline taxa. Oligosaline conditions continued between 3 100 and 1 500
cal. yr BP Around 1 500 cal. yr BP, there was a sharp rise in salinity, pro
bably accompanied by a shift to Na-CO3-type water, with marked seasonal and
interannual variability. Salinity decreased after 900 cal. yr BP, although
short-term variations were marked between 900 cal. yr BP and the top of th
e sequence, 95 cal. yr BP. Changes in the species assemblages and ostracod
abundance were a response to climate-driven variations in the seasonal and
interannual stability of the lake, together with changes in its salinity an
d solute composition, but there is no simple relationship between ostracod
faunas and salinity. Within Kajemarum, there is no evidence of ostracod ass
emblages typical of deep, fresh water, nor of hypersaline Na-Cl waters. The
sediments associated with the freshest waters at Kajemarum did not favour
ostracod preservation, and the driest climatic conditions were associated w
ith oligosaline to mesosaline water of Na-CO3-type. The species-poor assemb
lages reflect the short-term instability of the lake, coupled with the limi
ted opportunities for the colonisation of this isolated basin.