Award: OCE-1259043

The CARIACO Ocean Time-Series Program station program carried out 21 years of field work (November 1995-January 2017) in the Cariaco Basin in the southeastern Caribbean Sea off Venezuela. The CARIACO oceanographic station (located at 10.50°N-64.66°W) produced climate-quality data from highly productive surface waters, and from suboxic and anoxic habitats. The legacy of the program is knowledge about ocean chemistry, physics, biology, and geology, relating the ecology of the region to the settling of material to the bottom of the Cariaco Basin. Important scientific findings include: A longer period of coastal upwelling than previously reported, with sea surface temperatures between 23-25°C. A primary upwelling occurs between December-April. A second upwelling pulse occurs regularly in the middle of the year (June-August) over a shorter period (~5 weeks). In addition to seasonality, there is strong interannual variability in the upper 400 m in the Cariaco Basin. This is controlled by changes in the wind and in the intensity of the geostrophic Caribbean Current, reflecting large-scale changes in the Atlantic Basin. The seasonality and range of the mixed-layer depth varied little, from less than 10 m during the rainy season (August-October) to 35 m during the primary upwelling season (December-April). The length of the time-series was key to capture interannual variations, with some variability spanning over a decade. A decrease in upwelling intensity from 2003 to 2013 provided an opportunity to observe changes in the ecology of the Cariaco Basin. Changes led to an increase in zooplankton biomass and increased grazing pressure on the phytoplankton, which in turn resulted in a major shift in phytoplankton community composition and feedback loops that affected higher trophic levels, including the sardine fishery. A defining characteristic of the Cariaco Basin subsurface anoxia. The time series advanced our understanding of low-oxygen environments. Dissolved oxygen decreases with depth and reached values <5 μM between about 250 and 300 m. The depth of O2 disappearance and the depth of first appearance of H2S defined the boundaries of the oxic-anoxic interface. The thickness of this interface ranged 0-66 m, depending on ventilations. Cyclonic and anticyclonic eddies that interacted with the continental shelf can force denser, oxygenated water from the Caribbean Sea over the sill and into the Cariaco Basin, ventilating the waters bellow the oxycline (200-350 m) and stimulating biogeochemical reactions. CARIACO fully characterized the unique microbial community and structure of hypoxic and anoxic waters, and the processes that affect the composition and amplitude of the flux of particulate carbon to deeper waters and deposition on the ocean bottom. A novel ciliate class, Cariacotrichea, was identified based on phenotype and molecular phylogenies. The anaerobic decomposition of the settling organic matter particulate flux is as efficient as aerobic remineralization of organic matter. In spite of the high biological productivity in this region (320-628 g C m−2 y−1) and large vertical fluxes of particulate organic matter, only 1-3% of primary production fall to the bottom sediments at ~1310 m. The deepest waters of the Cariaco Basin (~1310 m) exhibited positive trends in temperature, salinity, hydrogen sulfide, ammonia, phosphate, methane, and silica. Temperature and salinity at the bottom are increasing due to the sinking of salty, warm water from the surface. Nutrients are increasing because of the continuing settling flux of organic particulates from the surface. Metabolism of organic matter is the process driving the increase of sulfide and methane; however, sulfide concentrations were highly diminished after water ventilations and after the dislodging of coastal sediments caused by earthquakes. CARIACO contributed to our understanding of ocean acidification. The station showed trends toward acidification (decreasing pH) comparable to those at other ocean time series around the world. The increasing trend of surface partial pressure of carbon dioxide (pCO2, +2.79 ± 0.37 μatm yr-1) was one of the highest measured. The decreasing trend in pH and increasing trend in dissolved inorganic carbon (DIC) concentration was observed from the surface to the bottom of the basin, with a higher rate of increase in deep waters. CARIACO advanced the development and refinement of paleoclimate proxies and helped improve the interpretation of climate signals recorded in the ocean′s sediments. Foraminiferal δ13C records from sediment cores extended the record of aquatic 13C concentrations back to the year 1700, showing changes in dissolved inorganic carbon in the marine carbon reservoir, including increases in anthropogenic CO2. Sea surface temperature in the Cariaco Basin had increased approximately 2°C since the end of the Little Ice Age. The Cariaco Basin had rapid millennial-scale changes in nitrogen cycling 35,000-55,000 years ago, synchronous with changes recorded in Greenland ice cores. The CARIACO data are publicly available via several Internet-based servers. The legacy of CARIACO will continue as its data and results are combined with future observations to help generate new discoveries. Last Modified: 07/03/2019 Submitted by: Frank E Muller-Karger