Abstract
Abstract. Changes in Southern Ocean export production have broad biogeochemical and climatic implications. Specifically, iron fertilization likely increased subantarctic nutrient utilization and enhanced the efficiency of the biological pump during glacials. However, past export production in the subantarctic southeastern Pacific is poorly documented, and its connection to Fe fertilization, potentially related to Patagonian Ice Sheet dynamics, is unknown. We report biological productivity changes over the past 400 kyr, based on a combination of 230Thxs-normalized and stratigraphy-based mass accumulation rates of biogenic barium, organic carbon, biogenic opal and calcium carbonate as indicators of paleo-export production in a sediment core upstream of the Drake Passage (57.5∘ S, 70.3∘ W). In addition, we use fluxes of iron and lithogenic material as proxies for terrigenous input, and thus potential micronutrient supply. Stratigraphy-based mass accumulation rates are strongly influenced by bottom-current dynamics, which result in variable sediment focussing or winnowing at our site. Carbonate is virtually absent in the core, except during peak interglacial intervals of the Holocene, and Marine Isotope Stages (MIS) 5 and 11, likely caused by transient decreases in carbonate dissolution. All other proxies suggest that export production increased during most glacial periods, coinciding with high iron fluxes. Such augmented glacial iron fluxes at the core site were most likely derived from glaciogenic input from the Patagonian Ice Sheet promoting the growth of phytoplankton. Additionally, glacial export production peaks are also consistent with northward shifts of the Subantarctic and Polar Fronts, which positioned our site south of the Subantarctic Front and closer to silicic acid-rich waters of the Polar Frontal Zone. However, glacial export production near the Drake Passage was lower than in the Atlantic and Indian sectors of the Southern Ocean, which may relate to complete consumption of silicic acid in the study area. Our results underline the importance of micro-nutrient fertilization through lateral terrigenous input from South America rather than eolian transport and exemplify the role of frontal shifts and nutrient limitation for past productivity changes in the Pacific entrance to the Drake Passage.
Highlights
The Southern Ocean (SO) plays an essential role in modulating glacial–interglacial variations of atmospheric pCO2 (Sigman et al, 2010)
Lithogenic content (232Th-derived, and that obtained by subtracting biogenic sediment components from the total bulk) and fueled by enhanced iron (Fe) show minima during these interglacials, and higher values during the rest of the record
Whereas in the central Pacific, the relatively low export production is explained by weaker Fe fertilization caused by lower local glacial dust fluxes (Lamy et al, 2014), the results at our site indicate that export production did not respond exclusively to glacial Fe fertilization (Fig. 6). Since it takes very little bio-available iron to get out of the Fe-limited conditions, and our core site is proximal to Fe sources, which might facilitate a faster transition from a Fe-depleted to Fereplete conditions, we suggest that under bio-available Fereplete conditions, another mechanism than Fe fertilization might regulate export production at our site
Summary
The Southern Ocean (SO) plays an essential role in modulating glacial–interglacial variations of atmospheric pCO2 (Sigman et al, 2010). Its flow is concentrated along several fronts, which are the Subantarctic Front (SAF), Polar Front (PF) and Southern ACC Front (SACCF; Orsi et al, 1995) These fronts act as barriers, inhibiting the exchange of the upwelled waters and their associated nutrients with neighboring fronts, and represent the limits between geochemical provinces (Chapman et al, 2020; Paparazzo, 2016). In some regions of the SO with weaker jets the mixing barrier effect is lower, allowing some degree of meridional exchange of nutrients and upwelled waters by eddy fields (Naveira Garabato et al, 2011) These SO fronts are not stationary and their positions have been shown to change on seasonal to orbital timescales (e.g., Gille, 2014; Kemp et al, 2010). At the DP entrance, a consistent pattern of glacial reduction of the ACC throughflow has been previously linked to a northward shift of the SAF (Lamy et al, 2015; Toyos et al, 2020)
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