Abstract The NW Arabian Sea is characterized by a strong seasonal contrast in surface water hydrography. During the SW monsoon of 1992, we encountered strong coastal upwelling characterized by low sea surface temperatures (SST), high nutrient concentrations, a shallow thermocline and a near-surface chlorophyll maximum. By contrast, the hydrography during the NE monsoon of 1993 was characterized by a relatively warm nutrient-depleted surface mixed layer and a deep chlorophyll maximum. We show that the faunal composition, depth habitat and abundance of living planktic foraminifera respond to the hydrographic changes controlled by the seasonally reversing monsoon system. Total shell concentrations (>125 μm) ranged from 4 to 332 individuals (ind.) m −3 during upwelling and from 3 to 85 ind. m −3 during the non-upwelling season. During upwelling, the fauna was dominated by Globigerina bulloides . During non-upwelling the fauna was characterized by relatively high concentrations of tropical symbiont-bearing species such as Globigerinoides ruber, Globigerinoides sacculifer and Globigerinella siphonifera, whereas concentrations of Globigerina bulloides were an order of magnitude lower. Factor analysis on 15 species yields an upwelling assemblage (UA), a tropical assemblage (TA) and a subsurface assemblage (SA). A fourth factor represents the distribution of the species Globigerina falconensis, which is mainly found in subsurface waters during the non-upwelling period (NE monsoon). A model is presented to calculate the base of the productive zone from the vertical shell concentration profile of a given species. The model is validated by comparing the range in calcification temperatures of G. bulloides, derived from its δ 18 O, with the in situ sea-water temperature range of the productive zone as predicted from the model. It appears that shell growth (calcite precipitation) is restricted to the productive zone as defined by this method. The average calcification temperature of G. bulloides corresponds to the point of maximum change in the shell concentration profile (i.e. the inflection point). For most shallow-dwelling species, the inflection point is found at or below the depth of the chlorophyll maximum, although above the main thermocline. This study indicates that the depth habitat and abundance of different species varies seasonally. Consequently, the abundance and stable isotope composition of specimens in the fossil record reflects a mixture of specimens that were produced at various depths during the different seasons.

Current radiometric dates of minerals in marine Neogene strata in California suggest a revision of the correlation between classic biostratigraphic units and the radiometric scale. The base of the lower Miocene corresponds approximately with the base of the Saucesian Stage, and it is dated at about 24.5 m.y.; the base of the middle Miocene is within the lower Relizian Stage, and, by extrapolation, it is slightly older than 19 m.y.; the base of the upper Miocene is equated with the base of the Mohnian Stage (the Globigerina pachyderma datum), and, again by extrapolation, it is no older than about 14 m.y.; the Miocene-Pliocene boundary is no older than about 9 m.y. and coincides approximately with the boundary between the Delmontian and Repettian Stages; the Pliocene-Pleistocene boundary is at least as old as 3 m.y.; and the Pleistocene-Holocene boundary is about 11,000 years B.P. Significant planktonic foraminiferal species of the California lower Miocene include an abundance of Globigerina concinna Reuss (with 5 chambers in the final whorl), and the presence of Catapsydrax stainforthi Bolli, Loeblich, and Tappan and Globorotaloides suteri Bolli. Abundant and significant planktonic species of the middle Miocene include Globigerina concinna var. (with 4.5 chambers in the final whorl) in the upper Relizian Stage and its replacement by an abundance of G. bulloides in the Luisian Stage. Globorotaloides trema Lipps appears to be restricted to the middle Miocene. Discoaster kugleri is a good index of the Upper Luisian. Primitive orbulines appear initially within the Lower Luisian, which is correlative with the Globorotalia praefohsi Zone. Upper Miocene planktonic events include the Globigerina pachyderma datum, the first appearance of significant populations of Globigerina pachyderma (Ehrenberg) ; this species continues to be an important and commonly dominant member of planktonic faunas throughout the remainder of the Neogene in California. “Orbulina universa” d’Orbigny, the Globorotalia menardii (d’Orbigny) group, Globigerinella siphonifera (d’Orbigny), and other important planktonic species make their initial appearances near the base of the upper Miocene. Rare specimens of “Sphaeroidinellopsis subdehiscens” (Blow) also occur within the upper Miocene. In addition, the radiolarian Prunopyle titan Campbell and Clark appears initially in the middle Miocene Luisian Stage, but it is most abundant and characteristic of the upper Miocene, disappearing at or below the upper boundary of the Miocene. The Miocene-Pliocene boundary in California is approximately equivalent to the Delmontian-Repettian boundary and is recognized by the transition from the Miocene forms of “ Sphaeroidinellopsis" to the forms of “Sphaeroidinella,” Pliocene-to-Holocene, which is one way of defining this boundary in tropical latitudes. The California lower Pliocene Repettian Stage is defined by the first appearance of Globorotalia puncticulata (d’Orbigny), Globorotalia inflata (d'Orbigny), and Globorotalia crassaformis (Galloway and Wissler). Upper Pliocene indices of California include the initial appearance of both Globorotalia truncatulinoides tosaensis Takayanagi and Saito and Globorotalia truncatulinoides truncatulinoides (d’Orbigny). Sinistrally coiled populations of Globigerina pachyderma, definitive of cold polar waters, define important cold cycles in the upper Miocene (approximately the Upper Mohnian Stage), the middle Pliocene, and within the Pleistocene.