Zeynep Erdem

Changes in diatom productivity and upwelling intensity off Peru since the Last Glacial Maximum: Response to basin‐scale atmospheric and oceanic forcing

New records of stable silicon isotope signatures (δ30Si) together with concentrations of biogenic opal and organic carbon from the central (9° S) and northern (5° S) Peruvian margin reveal changes in diatom productivity and nutrient utilization during the past 20,000 years. The findings are based on a new approach using the difference between the δ30Si signatures of small (11-32μm) and large (>150μm) diatom fractions (Δ30Sicoscino-bSi) in combination with the variance in diatom assemblages for reconstruction of past upwelling intensity. Combination of our records with two previously published records from the southern upwelling area off Peru (12-15° S) shows a general decoupling of the environmental conditions at the central and southern shelf mainly caused by a northward shift of the main upwelling cell from its modern position (12-15° S) towards 9° S during Termination 1. At this time only moderate upwelling intensity and productivity levels prevailed between 9° S and 12° S interpreted by a more northerly position of Southern Westerly Winds and the South Pacific Subtropical High. Furthermore, a marked decrease in productivity at 12-15° S during Heinrich Stadial 1 coincided with enhanced biogenic opal production in the Eastern Equatorial Pacific, which was induced by a southward shift of the Intertropical Convergence zone and enhanced northeasterly trade winds. Modern conditions were only established at the onset of the Holocene. Past changes in preformed δ30Si signatures of subsurface waters reaching the Peruvian Upwelling System did not significantly affect the preserved δ30Si signatures.

Pleistocene to Holocene benthic foraminiferal assemblages from the Peruvian continental margin

The benthic foraminiferal inventory and their assemblage composition was documented along five sediment cores from the Peruvian margin between 3°S and 18°S at water depths of 500 to 1250 m, covering the lower boundary of today’s Oxygen Minimum Zone (OMZ). Emphasis was given to certain time intervals during the last 22 thousand years when different climatic and oceanographic conditions prevailed than today. In total three agglutinated and 186 calcareous species were recognised. Bolivina costata, Bolivinita minuta, Cassidulina delicata and Epistominella exigua were most abundant. The foraminiferal distributions revealed a marked change in assemblage composition particularly at the deeper cores during and after the deglaciation. The diversity declined and Bolivina species became dominant. These changes took place gradually over several millennia, and high-frequency fluctuations were not recorded. This pattern provides evidence for rather stable ecological conditions and sluggish changes in bottom water circulation during the last deglaciation.

A 22 kyr record of an evolving mid-depth hiatus in sediments at the Peruvian margin

The Peruvian coastal region has long been in the focus of marine geological investigations because of its importance to understand high productivity areas and the oxygen minimum zones (OMZs) in today’s and past oceans. The reconstruction of paleoenvironmental conditions for periods since the Last Glacial Maximum was hampered by a ubiquitous hiatus in sediment core records. We combined the stratigraphical information of 22 sediment cores from the literature and own results from 9 cores collected during R/V Meteor cruises M77/1 and M77/2 as a part of Sonderforschungsbereich 754. The cores were located between 3° and 18S and water depths of 90 to 1300 m within and below the Peruvian OMZ. In general, Peruvian Margin sediments consisted of olive green to greyish green silty clays predominantly showing laminations within the OMZ. Diatomaceous oozes were occasionally found underneath the main upwelling areas. Homogenously bioturbated silty clays with planktonic foraminifera were found around the OMZ in particular in the northern part of the region. Cores obtained from south of 7S showed slumping, erosional surfaces, phosphorite sands and unconformities. In order to investigate the distribution of the hiatus in space and time, we compared the lithologies of the cores of the corresponding time intervals; Late Holocene, Early Holocene, Bølling/Allerød, HeinrichStadial 1 and Last Glacial Maximum with the Recent conditions. Each time interval showed abundant unconformities successively progressing along the continental slope from south to north during the deglaciation. It has been suggested that the erosion and concomitant non-deposition was caused by the poleward undercurrent (PCUC: Peru Chile Undercurrent) (Reimers and Suess, 1983; Reinhardt et al., 2002 and the references therein) which today feeds the upwelling in the region. The PCUC originates around 5-7°S and is centred between 50 and 400 m water depth, with a well-defined core around 100 to 300 m. The undercurrent reaches its highest velocities around 10°S and leads to partial erosion along the shelf. In addition to the PCUC, breaking internal waves affect sedimentation and shape the slope at greater depths between 500 and 700 m around 11S (Mosch et al., 2012). Both, undercurrent and internal waves created erosional surfaces, non-deposition and slumps, evolving and affecting a wider area with the onset of Termination I.