The Indian monsoon system, as recorded by ocean-floor biota (benthic foraminifera) at Ocean Drilling Program Site 758 in the eastern equatorial Indian Ocean, has varied dramatically over the past 5.5 m.y., long after the onset of the monsoons at 10-8 Ma. Benthic foraminifera that thrive with high productivity year-round were common before the formation of Northern Hemisphere continental ice sheets ca. 3.1-2.5 Ma, indicating that the summer (southwest) monsoon had high intensity and long seasonal duration. Ca. 2.8 Ma benthic faunas became dominated by taxa that flourish with a seasonally strongly fluctuating food supply, indicating that the northeast (winter) monsoon, during which primary productivity is relatively low, increased in duration and strength to form a system similar to that of today. The change occurred coeval with the initiation of the Northern Hemisphere glaciation, documenting a close link between the development of the Indian monsoon and Northern Hemisphere glaciation.

Benthic foraminiferal species and biofacies:Gs (Factor 1 positive scores)Globocassidulina subglobosa (P)(0.70994) Oridorsalis umbonatus (P)(0.63178)Pullenia bulloides (S)(0.51520)Cibicides kullenbergi (S)(0.39229)Environment - relatively warm, intermediate organic flux, intermediate seasonality, moderate oxygenationUp (Factor 1 negative scores)Uvigerina proboscidea (P)(-0.77203)U. hispido-costata (=peregrina ) (S)(-0.34283)Environment - continuous, high organic flux, low seasonalitySa (Factor 5 positive scores)Stilostomella abyssorum (P)(0.63951)Melonis pompilioides (P)(0.48769)Cibicides bradyi (S)(0.39493)Pleurostomella brevis (S)(0.39042)P. obtusa (S)(0.29295)Environment - high-moderate organic flux, intermediate seasonalityMb (Factor 10 positive scores)Melonis barleeanum (P)(0.71888)Dentalina stimulea (S)(0.44626)Pleurostomella acuminata (S)(0.32599)P. obtusa (S)(0.30844)P. brevis (S)(0.30620)Environment - intermediate organic flux, intermediate to high seasonality, refractory organic matterEe (Factor 5 negative scores)Epistominella exigua (P)(-0.47785)Laticarinina pauperata (S)(-0.24486)Pullenia osloensis (S)(-0.37974)P. salisburyi (S)(-0.32038)Environment - cool, strongly pulsed, low to intermediate organic flux, high seasonalityNu (Factor 11 positive scores)Nuttallides umbonifera (P)(0.68068)Eggerella bradyi (S)(0.35639)Environment - cool, carbonate corrosive, organic flux variable, high oxygenationPm (Factor 9 positive scores)Pyrgo murrhina (P)(0.82297)Karreriella bradyi (S)(0.54175)Quinqueloculina lamarckiana (S)(0.23744)Environment - cool, strongly pulsed organic flux, high oxygenation, high seasonalityCc (Factor 3 positive scores)Cassidulina carinata (P)(0.80245)Gyroidinoides nitidula (P)(0.76187)Pullenia bulloides (S)(0.36517)Sphaeroidina bulloides (S)(0.29619)Environment - intermediate organic flux, intermediate to high seasonalityCw (Factor 8 positive scores)Cibicides wuellerstorfi (P)(0.72310)Sphaeroidina bulloides (S)(0.39942)Anomalina globulosa (S)(0.39310)Environment - cool, active currents, low-intermediate organic flux, high seasonality, high oxygenationNote: In parentheses: factors and factor scores for each biofacies and species.P = primary species, S = secondary species.

Supplement to: Gupta, Anil K; Thomas, Ellen (2003): Initiation of Northern Hemisphere glaciation and strengthening of the northeast Indian monsoon: Ocean Drilling Program Site 758, eastern equatorial Indian Ocean. Geology, 31(1), 47-50