Changes in circulation and trophic levels in the Pliocene Caribbean Sea: Evidence from benthic foraminifer accumulation rates

Changes in benthic foraminifer faunas throughout the late Neogene (about 5. 8-1.8 Ma) were analyzed in DSDP Hole 502A (Caribbean Sea) to determine whet her the development of the Isthmus of Panama and resulting changes in botto m-water circulation affected the benthic foraminifer community. Benthic for aminifer accumulation rates (BFAR) of the 11 most abundant and presumably a lso ecologically significant species revealed three intervals of distinct f aunal developments: Interval 1 (prior to 4.65 Ma) exhibits a fluctuating pa ttern in the benthic foraminifer fauna with an increase of Epistominella ex igua between 5.7 and 5.35 Ma, except at about 5.4 Ma. This variation of E. exigua may indicate a period of increased vertical flux of organic (phytode tritial) matter to the seafloor at the base of the sequence. Also towards t he upper part of Interval 1, E. exigua shows periods of higher abundance, w hich could be related to higher vertical flux of phytodetritus to the seafl oor. Interval 2 (4.65 to 3.9 Ma) is marked by a gradual increase of C. wuel lerstorfi and Oridorsalis umbonatus, and decrease of Nuttallides umbonifera with periods of higher abundance of E. exigua. This faunal change can be r elated to alternations of sudden phyto-detritus inputs and increased circul ation within the Caribbean Sea that resulted from the progressive emergence of the Panamanian landbridge changing the Caribbean Sea from a broad ocean ic seaway into a marginal sea. The restricted surface-water how over the Is thmus of Panama probably enhanced northward transport of warm, high-salinit y waters into the high latitudes via the Gulf Stream and thus stimulated th e total production of North Atlantic Deep Water (NADW) leading to an increa sed inflow of Upper North Atlantic Deep Water (UNADW) into the Caribbean Se a. The increased bottom-water activity in the Caribbean may have favored C. wuellerstorfi, which has been found to prefer an elevated suspension feedi ng position above the sediment surface. Intensified bottom-water circulatio n would allow more water to pass and thus provide more available food for t his particular species. Interval 3 (about 3.9 to 1.8 Ma) began with a strik ing decrease of C. wuellerstorfi coeval with a rapid increase of N. umbonif era, which became the dominating species. This may have been a response to a declined velocity of the bottom-water currents in the Caribbean, probably caused by less inflow of bottom waters from the North Atlantic. The organi c flux into the area may have been similar to Interval 2, but lower bottom- water current velocities may have favored the more oligotrophic species N. umbonifera relative to C. wuellerstorfi. The peak abundance of E. exigua be tween about 3.55 and 3.45, and at 3.4-3.35 Ma may be a result of strong but interrupted inputs of phyto-detritus into the Colombia Basin. Also in the upper part of the interval particularly between approximately 2.5 and 2.2 M a the abundance of E. exigua exhibit increased values. At about 3.0 Ma N. umbonifera shows a drastic decrease and coeval recovery of C. wuellerstorfi, O. umbonatus, and Pyrgo murrhina. This faunal change c ould be attributable to (a) mixing between the base of nutrient-rich Antarc tic Intermediate Water (AAIW) and the upper layer of Upper North Atlantic D eep Water (UNADW), and/or (b) nutrient-rich local river outflow (e.g. from the Rio Magdalena) together with, at least periodically, (9) increased bott om-water currents that favored the normally elevated and suspension feeding C. wuellerstorfi. The short term alternation in the benthic foraminifer ab undance, i.e. the instant recovery of N. umbonifera in the lower part of th e interval, may indicate an amelioration of deep-water conditions, which ma y have been associated with a slower inflow of bottom water into the Caribb ean Sea. Moreover, the increased average benthic delta(13)C value during th e upper part of Interval 3 may also be a result of a better bottom-water ve ntilation in the Colombia Basin linked to the onset of the modern deep-wate r circulation, which most likely is related to periodically increased inflo w of UNADW into the Caribbean Sea.