Abstract:
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 whether the development of the Isthmus of Panama and resulting changes in bottom-water circulation affected the benthic foraminifer community. Benthic foraminifer accumulation rates (BFAR) of the 11 most abundant and presumably also ecologically significant species revealed three intervals of distinct faunal developments: Interval 1 (prior to 4.65 Ma) exhibits a fluctuating pattern in the benthic foraminifer fauna with an increase of Epistominella exigua 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 (phytodetritial) matter to the seafloor at the base of the sequence. Also towards the upper part of Interval 1, E. exigua shows periods of higher abundance, which could be related to higher vertical flux of phytodetritus to the seafloor. Interval 2 (4.65 to 3.9 Ma) is marked by a gradual increase of C. wuellerstorfi and Oridorsalis umbonatus, and decrease of Nuttallides umbonifera with periods of higher abundance of E. exigua. This faunal change can be related to alternations of sudden phyto-detritus inputs and increased circulation within the Caribbean Sea that resulted from the progressive emergence of the Panamanian landbridge changing the Caribbean Sea from a broad oceanic seaway into a marginal sea. The restricted surface-water flow over the Isthmus of Panama probably enhanced northward transport of warm, high-salinity waters into the high latitudes via the Gulf Stream and thus stimulated the total production of North Atlantic Deep Water (NADW) leading to an increased inflow of Upper North Atlantic Deep Water (UNADW) into the Caribbean Sea. The increased bottom-water activity in the Caribbean may have favored C. wuellerstorfi, which has been found to prefer an elevated suspension feeding position above the sediment surface. Intensified bottom-water circulation would allow more water to pass and thus provide more available food for this particular species. Interval 3 (about 3.9 to 1.8 Ma) began with a striking decrease of C. wuellerstorfi coeval with a rapid increase of N. umbonifera, 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 organic 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 between 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 Ma 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 could be attributable to (a) mixing between the base of nutrient-rich Antarctic Intermediate Water (AAIW) and the upper layer of Upper North Atlantic Deep Water (UNADW), and/or (b) nutrient-rich local river outflow (e.g. from the Rio Magdalena) together with, at least periodically, (c) increased bottom-water currents that favored the normally elevated and suspension feeding C. wuellerstorfi. The short term alternation in the benthic foraminifer abundance, i.e. the instant recovery of N. umbonifera in the lower part of the interval, may indicate an amelioration of deep-water conditions, which may have been associated with a slower inflow of bottom water into the Caribbean Sea. Moreover, the increased average benthic delta (super 13) C value during the upper part of Interval 3 may also be a result of a better bottom-water ventilation in the Colombia Basin linked to the onset of the modern deep-water circulation, which most likely is related to periodically increased inflow of UNADW into the Caribbean Sea.