Mesoscale circulation in the Guaymas Basin (Gulf of California) and its role in paleoproductivity during the past 31 ka

Abstract

<p>Mesoscale geostrophic vortices are an important oceanographic feature in the Guaymas Basin (GB). Depending on the depth, bathymetry, coastline, local wind, and surface current conditions, they exhibit different behavior. Their size and direction of rotation are forced mainly by the Eastern Tropical Pacific Ocean (ETPO), with some contribution from monsoonal winds. Cyclonic vortices cause divergence, advecting nutrients into the photic zone and stimulating productivity; in contrast, anticyclonic vortices induce convergence at the surface and produce down-welling and surface layers poor in nitrates and chlorophylls, but they transport and disperse upwelled waters enriched in nutrients from the east to the west side. Anticyclonic vortices dominate the circulation in October, but vortices of either sign can be present from November to March. </p><p>                        This research aims to investigate productivity fluctuations in the west side of the Guaymas Basin, understand the role of mesoscale vortices and the link with a mesoscale remote forcing in the ETPO during the past 31,000 yr based on biogenic opal and Ba/Ti ratio records in Hole 1545A (~42 mbsf) of IOPD Expedition 385. This period comprises significant climatic events, including the Last Glacial Maximum (LGM), the deglacial to Holocene transition, the Holocene thermal maximum (HTM), and changes in North America Monsoon (NAM).</p><p>            The age model was based on 17 radiocarbon AMS analyses on bulk sediments. We used a Bayesian statistics approach with the R package “Bacon” using the Marine 20 calibration curve and a regional reservoir-age correction (∆R) 301 ± 50 yr. The records show an increase in productivity from ~ 31,000 to 28,300 yr in the early stage of MIS 2, which then declines markedly during the LGM (26,000 to 19,000 yr), reaching its minimum value of ~25,000 yr. This pattern might be related to the expansion of the Polar Cell that caused the southerly migration of the North Pacific High (NPH) system, weakening the vortices, changing NW wind strengths, and reducing wind-driven upwelling in the eastern margin, causing low productivity. In addition, at that time, the poleward coastal circulation transported nutrient-poor waters from the ETPO. During the deglaciation, records show increased productivity, with high opal values between 16,000 and 15,000 yr. A gradual increase in productivity is observed from the glacial to the Holocene, particularly during HTM between 10,500 and 8,000 yr. From ~ 7,000 to 2,200 yr, a gradual productivity decrease might be related to the NAM onset (~ 6,000 yr). For the last 3,000 cal yr BP productivity increased, but with important fluctuations. We hypothesize that productivity in the northwestern Guaymas Basin during the last 31,000 yr has been controlled mainly by mesoscale vortices, linked with a mesoscale remote forcing in the tropical Pacific coast and with a local mesoscale forcing of the wind, rather than by upwelling.</p>