Upwelling and primary production during the U.S. GEOTRACES East Pacific Zonal Transect

Abstract

The 2013 U.S. GEOTRACES Eastern Pacific Zonal Transect (EPZT) traversed the highly productive Peruvian coastal upwelling (PCU) region. In this work, the flux of nitrate into the euphotic zone is derived for stations within the PCU using a previously developed method whereby dilution of the water column 7Be inventory by upwelled 7Be‐free water provides a means to infer upwelling rates. Furthermore, with knowledge of upwelling rates, 7Be profiles are used to constrain vertical diffusivity within the upper thermocline. These transport terms are applied to nitrate profiles to estimate net community production between 79°W and 104°W along the EPZT, which includes the zone of active upwelling to the edge of the oligotrophic gyre. With a simple, one‐dimensional model, the calculated upwelling rates were inversely related to mixed layer temperature and ranged from 0 to 3.0 m/d. Results using a depth‐dependent upwelling rate with a component of horizontal advection are also described. Vertical diffusivities near the base of the euphotic zone were in the range 1.7–4.5 × 10−4 m2/s. These values are compared to those generated by analysis of temperature profiles. Net community production averaged 15 mmol C/m2/d for stations between 84°W and 104°W and was 134 mmol C/m2/d for the furthest inshore station at 79°W which displayed the lowest SST and greatest rate of upwelling.

An investigation of basin-scale controls on upper ocean export and remineralization

“…Table 3). Kadko (2017) and other prior studies (Haskell et al, 2015;Kadko and Johns, 2011) determine upwelling rates using the dilution of the 7 Be inventory in the surface ocean by upwelling of 7 Be-free waters from below. 7 Be-derived upwelling rates are well suited for use with 234 Th because of the comparable half-life (~53 days) and 7 Be-derived rates will reflect trends observed over a greater portion of the euphotic zone than tracers like helium (Haskell et al, 2015).…”

“…7 Be-derived upwelling rates are well suited for use with 234 Th because of the comparable half-life (~53 days) and 7 Be-derived rates will reflect trends observed over a greater portion of the euphotic zone than tracers like helium (Haskell et al, 2015). A 1-D model was used by Kadko (2017) to determine the 7 Be-derived upwelling rate at the base of the mixed layer for cases of invariant upwelling with depth (singular, constant value) and depthdependent upwelling (linear decrease of upwelling rate with depth).…”

“…2. However, the locations between 77º W and 84º W were determined to be impacted by upwelling (Kadko, 2017) and all 234 Th fluxes used here for this region have been corrected for these effects . The high-resolution measurements of total 234 Th allow for the calculation of 234 Th flux at any depth in the upper 400 m by linear extrapolation between sampled depths.…”

“…For these stations, Ez was shallower than the gradient boundary. Upwelling rates from Kadko (2017) Better constraints on the magnitude of surface export, the extent of remineralization, and the residence time of biologically-relevant elements are required in order to understand marine food web dynamics, track the transport of anthropogenic trace metals in the ocean, and improve global climate models. While prior studies have been successful in constructing basin-scale budgets of carbon in the surface ocean, the cycling of particulate trace metals is poorly understood.…”

Insights From the238U‐234Th Method Into the Coupling of Biological Export and the Cycling of Cadmium, Cobalt, and Manganese in the Southeast Pacific Ocean

“…For most stations, the particulate flux of 234 Th was found using equation (2). However, the locations between 77 and 84°W were determined to be impacted by upwelling (Kadko, 2017) and all 234 Th fluxes used here for this region have been corrected for these effects (Black et al, 2018). The high-resolution measurements here allow for the calculation of 234 Th flux at any depth by linear extrapolation between sampled depths.…”