High-latitude controls of thermocline nutrients and low latitude biological productivity

Abstract

The ocean's biological pump strips nutrients out of the surface waters and exports them into the thermocline and deep waters. If there were no return path of nutrients from deep waters, the biological pump would eventually deplete the surface waters and thermocline of nutrients; surface biological productivity would plummet. Here we make use of the combined distributions of silicic acid and nitrate to trace the main nutrient return path from deep waters by upwelling in the Southern Ocean and subsequent entrainment into subantarctic mode water. We show that the subantarctic mode water, which spreads throughout the entire Southern Hemisphere and North Atlantic Ocean, is the main source of nutrients for the thermocline. We also find that an additional return path exists in the northwest corner of the Pacific Ocean, where enhanced vertical mixing, perhaps driven by tides, brings abyssal nutrients to the surface and supplies them to the thermocline of the North Pacific. Our analysis has important implications for our understanding of large-scale controls on the nature and magnitude of low-latitude biological productivity and its sensitivity to climate change.

Opal burial in the equatorial Atlantic Ocean over the last 30 ka: Implications for glacial‐interglacial changes in the ocean silicon cycle

“…Iron limitation is of consequence because it increases the relative uptake rate of Si and N by diatoms to (4:1) [Franck et al, 2000;Pondaven et al, 2000] compared to the value of 1:1 under nutrient-replete conditions [Brzezinski, 1985]. This relatively greater depletion of silicic acid combined with preferential regeneration of N (relative to Si) from sinking organic debris [Nelson et al, 2002] results in the export of surface waters to the Subantarctic zone with low concentrations of Si relative to N [Sarmiento et al, 2004]. North of the APF these Si-depleted surface waters mix with deeper waters to form Sub-Antarctic Mode Water (SAMW) (Pacific basin) or South Atlantic Central Water (SACW) (Atlantic basin), which sink and fill the thermocline of the southern hemisphere oceans, eventually upwelling at the equator [Toggweiler et al, 1991;Sigman et al, 1999;Stramma and England, 1999;Brzezinski et al, 2002].…”

“…North of the APF these Si-depleted surface waters mix with deeper waters to form Sub-Antarctic Mode Water (SAMW) (Pacific basin) or South Atlantic Central Water (SACW) (Atlantic basin), which sink and fill the thermocline of the southern hemisphere oceans, eventually upwelling at the equator [Toggweiler et al, 1991;Sigman et al, 1999;Stramma and England, 1999;Brzezinski et al, 2002]. These mixtures feeding the thermocline have dissolved Si:N ratios of about 1:2 [Sarmiento et al, 2004]. Waters upwelled in the equatorial Pacific and Atlantic Oceans have different source regions within the Southern Ocean.…”

“…[13] In their discussion of the SALH, Sarmiento et al [2004] suggested that the greatest increased in equatorial opal burial should be in the Eastern Equatorial Pacific (EEP), as this is where the strongest signal of SO waters is found today [Toggweiler et al, 1991]. Two studies have specifically tested the SALH in this area using downcore opal burial records [Bradtmiller et al, 2006;Kienast et al, 2006b].…”

Paleonutrient and productivity records from the subarctic North Pacific for Pleistocene glacial terminations I to V

“…[6] However, massive nutrient supply may also result from a warming of subarctic surface waters and/or reduced freshwater advection, leading to a lowered precipitation/ evaporation balance that in turn promotes vertical mixing across a weakened North Pacific pycnocline, as suggested by Sarmiento et al [2004]. In contrast to the first model this process will lead to reversed THC in the far northwestern Pacific Ocean, a scenario recently confirmed by extremely low planktic and benthic 14 C reservoir ages in the subarctic North Pacific for most of Termination Ia and later on [Sarnthein et al, 2007[Sarnthein et al, , 2004.…”

Recent Changes in the Ventilation of the Southern Oceans

“…T he transport of surface waters into the interior ("ventilation") of the southern oceans plays an important role in global climate and the cycling of carbon, oxygen, and nutrients in the oceans (1)(2)(3). Over the past few decades, the southern oceans have warmed at roughly twice the rate of the global mean ocean (1), and around 40% of the anthropogenic carbon in the oceans entered south of 40°S (4).…”