Description: 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 Southeast 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 ka, 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 (57.5º S; 70.3º 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 aeolian transport, and exemplify the role of frontal shifts and nutrient limitation for past productivity changes in the Pacific entrance to the Drake Passage.

Description: The Antarctic Circumpolar Current (ACC) is the world's largest current system connecting all three major basins of the global ocean. Our knowledge of glacial‐interglacial changes in ACC dynamics in the southeast Pacific is not well constrained and presently only based on reconstructions covering the last glacial cycle. Here we use a combination of mean sortable silt grain size of the terrigenous sediment fraction (10–63 μm, ) and X‐ray fluorescence scanner‐derived Zr/Rb ratios as flow strength proxies to examine ACC variations at the Pacific entrance to the Drake Passage (DP) in the vicinity of the Subantarctic Front. Our results indicate that at the DP entrance, ACC strength varied by ~6–16% on glacial‐interglacial time scales, yielding higher current speeds during interglacial times and reduced current speeds during glacials. We provide evidence that previous observations of a reduction in DP throughflow during the last glacial period are part of a consistent pattern extending for at least the last 1.3 Ma. The orbital‐scale cyclicity follows well‐known global climate changes from prevailing ca. 41‐kyr cycles in the early part of the record (1.3 Ma to 850 ka; marine isotope stage 21) across the mid‐Pleistocene transition into the middle and late Pleistocene 100‐kyr world. A comparison to a bottom water flow record from the deep western boundary current off New Zealand (Ocean Drilling Program Site 1123) reveals anti‐phased changes between the two sites. The enhanced supply of deep water along the DP and into the Atlantic Ocean during interglacials corresponds to a weakened flow of the SW Pacific deep western boundary current.

Description: Alkenone unsaturation indices (Uk'37) of marine sediment could prove particularly useful on organic-rich continental margins where carbonate dissolution hampers the use of other paleoclimatic proxies [McCaffrey et al., 1990, doi:10.1016/0016-7037(90)90399-6; Kennedy and Brassell, 1992, doi:10.1016/0146-6380(92)90040-5]. Forty core top samples of Recent sediment from a latitudinal transect (23°-40°N) along the California margin yield Uk'37 values that correlate linearly with modern mean annual sea surface temperatures (SSTs) in the range of 12°-23°C. Reproducibility of the unsaturation value in closely spaced cores is near analytical error. Uk'37 data define a relationship to temperature nearly identical to the Prahl et al. [1988, doi:10.1016/0016-7037(88)90132-9] laboratory cultures of Emiliania huxleyi. The close agreement is particularly significant in light of the nannofossil composition of the sediments, where the abundance of the coccolith taxon Gephyrocapsa oceanica (known to synthesize alkenones) equals or exceeds that of E. huxleyi. Comparison with seasonal temperature variations at different depths indicates that little if any alkenone production occurs at depths 〉30 m along the continental margin (water depths 〈2 km). Sediments in more pelagic locations exhibit small but consistent biases toward winter and/or subsurface production similar to previously reported sediment trap and core top data from the Oregon margin [Prahl et al., 1993, doi:10.1016/0967-0637(93)90045-5; Doose et al., 1997, doi:10.1029/97PA00821].

Description: Holocene carbon accumulation rates from three sediment records from bays Trampa, Caribe and Churruca (sites BT, BC and CHU) in the western Magellan fjord system.