First record of middle Miocene marine diatoms from the Colombian Pacific (NW South America) and their paleoceanographic significance

Middle Miocene marine diatoms have been reported in the eastern equatorial Pacific by the Deep Sea Drilling Project (DSDP), the Ocean Drilling Program (ODP), and the Integrated Ocean Drilling Program (IODP); few studies, however, have been completed on onshore sections from South America. Vallejo et al. (2006) published the biostratigraphy of the Ladrilleros-Juanchaco section on the northwest Pacific Colombian coast utilizing calcareous nannofossils, planktonic foraminifera and diatoms. In this work we present in detail the study of marine diatoms of that section in order to understand their implications with the age model proposed and provide a paleoceanographic interpretation. The diatom recovery is restricted to two intervals at the top of the Miocene sequence. Two biozones from the low latitude zonation can be recognized; from base to top they are: 1) The Coscinodiscus lewisianus Zone, which contains the same species assemblage reported in Antofagasta coastal sediments, Chile; and 2) the Craspedodiscus coscinodiscus Zone. We found that the diatoms represent older ages than proposed by calcareous nannofossils. These different ages suggest diachronism between the events of microfossil groups. Nevertheless, in a new interpretation of the distribution patterns of calcareous nannofossils in the upper part of the section, the diatom age model could be synchronous. The high abundance of Thalassionema nitzschioides and Thalassiothrix longissima reveal cool, upwelling-rich oceanic waters at the top of the sequence, as well as an increase in paleoproductivity suggested by an increase of Rhizosolenia spp. and T. longissima mats. Cool water trends support a strengthening of the Peru-Chile current. Actinoptychus sp., Leptocylindrus danicus and Paralia sulcata point to a nearshore environment. The common occurrence of fresh water continental diatoms and palm silicophytoliths indicate an increased input of continental material. Since ichnofossils and benthic foraminifera reveal ocean depths below 1000 m, this change may have related to hyperpycnal flows.