Effects of horizontal advection and vertical mixing on natural iron fertilization in Southern Drake Passage

• Main Author: Frants, Marina
• Format: Thesis
• Language: English
• Published: University of California, San Diego 2011
• Subjects: Physical oceanography; Antarctic; Antarctic Peninsula; Drake Passage; Ona Basin; Shackleton; Shackleton Fracture Zone; South Shetland Islands; Southern Ocean; The Antarctic; Antarc*
• Online Access: http://pqdtopen.proquest.com/#viewpdf?dispub=3457451

A combination of in situ and satellite data is used to examine the effects of horizontal advection, diapycnal mixing, and mixed-layer entrainment on natural iron fertilization in a region near the Shackleton Fracture Zone in southern Drake Passage. Results from Optimal Multiparameter analysis of temperature, salinity, oxygen and nutrient data are consistent with a scenario in which iron-rich shelf waters from the South Shetland Islands and the Antarctic peninsula are advected northward on the eastern side of the Shackleton Transverse Ridge (STR), where they interact with the low-iron waters of the Antarctic Circumpolar Current (ACC) in the Ona Basin. However, comparisons between sea surface height and Chlorophyll-a (Chl-a) levels in the region for the years 1997–2010 indicate that this advection process is significantly correlated with biological productivity only during the months of November and December, while high Chl-a concentrations persist through March. Enhanced diapycnal mixing and mixed-layer entrainment are considered as alternative mechanisms for delivering iron into the Ona Basin mixed layer. While fine-scale mixing estimates lack the vertical resolution to fully resolve the small-scale density fluctuations in the low-stratification regime of the Southern Ocean, microstructure measurements indicate that diapycnal diffusivities in Drake Passage are on the order of 10−4 m2 s−1. Based on these diffusivities, estimates of iron flux from iron concentrations measured during the summer of 2004 and winter of 2006 suggest that diapycnal mixing alone can supply iron ito the mixed layer at a rate of approximately 96±9 nmol m −2 day−1 during the winter and approximately 64±2 nmol m−2 day−1 during the summer. In addition, the Ona Basin mixed layer deepens from January into April, allowing for iron to be steadily entrained from below. Simulation of these vertical processes with a simple one-dimensional model yields an estimated mixed-layer entrainment rate of 12±9 nmol m−2 day−1, while the combined effect of mixing and entrainment can supply 176±44~nmol m−2 day−1 into the top 30 m from January to April, which is sufficient to sustain the phytoplankton bloom that persists in the basin throughout the summer months.