Böschen, Tebke (2013) Planktonic and Benthic Foraminifers as Geochemical Proxies Recording Hydrographic Changes in the Eastern Equatorial Pacific. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, 188 pp.

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Abstract

Oxygen Minimum Zones (OMZs) in the ocean represent key regions for the interaction between atmosphere and ocean waters and can be a sink or source for atmospheric CO2. Hence, they play a major role with respect to anthropogenic induced global warming. Biological productivity is very high in these areas and resulting degradation processes consume substantial amounts of dissolved oxygen from the water column, leaving certain water depths almost oxygen‐free. One of the largest and most distinctive OMZs worldwide stretches along the west coast of South America offshore Ecuador, Peru and Chile. This OMZ is associated with intense coastal upwelling, which is most pronounced between ~5°S and ~25°S. The intensity of the upwelling varies seasonally, being propelled mainly by external forcing mechanisms like the position of the Intertropical Convergence Zone (ITCZ), the strength of trade winds and modulations of the El Niño Southern Oscillation (ENSO). This hydrographically complex ocean area affords different approaches to study its characteristics. Sediment surface samples and corresponding seawater samples from the Eastern Equatorial Pacific (EEP), ranging from a water depth of 60 to 2,000 m and covering a large latitudinal range from the equator to 17°S, were investigated. Apparent calcification depths (ACDs) were calculated by combined analyses of stable oxygen isotopes (d18O) in planktonic foraminifers from these samples and from water samples (Chapter 4). Investigations of ACDs for species G. ruber (white, morphotype sensu stricto) and N. dutertrei (dextral) show clear differences between both species within the EEP. Surface dweller G. ruber constantly inhabits the upper 16 meters of the water column; contrary N. dutertrei favors different habitat depths in dependence on the surrounding water masses. On average, the ACDs for N. dutertrei indicate a calcification depth of ~200m in the northern part of the study area (the equator to 5°S) and ~70m in the southern region (7° to 13°S). The shallower habitat depth in the south is probably related to the more intense upwelling, which results in the rise of cooler water masses and thus similar temperatures (~15°C) are reached in shallower depths. Additionally, lower oxygen concentrations in the southern part of the study area are likely to force N. dutertrei to shallower habitat depths. The correct estimation of ACDs in the EEP provides a more detailed view on the vertical habitat of N. dutertrei, which is particularly important for the following paleointerpretations in Chapter 5. Two piston cores from the Gulf of Guayaquil at 3.5°S in the EEP were studied, covering time spans of ~13,000 years (core 056; temporal resolution ~60 years) and of ~18,000 years (core 059; temporal resolution ~80 years), respectively (Chapter 5). Surface and subsurface temperatures were reconstructed for the past 18,000 years using alkenone thermometry as well as foraminiferal Mg/Ca‐based approaches for surface (Globigerinoides ruber, white) and for subsurface depths (~200 m; Neogloboquadrina dutertrei). Salinity approximations (d18Oivf‐sw) for surface and subsurface depths were generated to corroborate the high‐resolution temperature records. In the study area the deglaciation was characterized by much warmer temperatures (~3°C) and higher salinities than nowadays, in surface as well as in subsurface depths. Surface dweller G. ruber and alkenone records show almost identical temperatures prior to ~10 ka BP, indicating that both proxy carriers recorded a similar habitat and seasonality during this time. After ~10 ka BP both proxies divert markedly, which was possibly caused by a shift of alkenone‐synthesizing coccolithophores from annual mean temperatures during the deglaciation to austral summer temperatures during the Holocene. The cooling in G. ruber records by ~2°C in both cores during the Holocene implies a strengthened influence of cooler water masses. The surface records lag the subsurface records of N. dutertrei by ~1‐2 kyrs, implying that different mechanisms control the surface and subsurface hydrography. The temperature decrease and freshening trends during the Holocene coincide with an enhanced marine productivity, indicating that cooler (EUC‐sourced) water masses and strengthened upwelling intensity transported more nutrients into the EEP. A newly developed species‐specific transfer function for benthic Mg/Ca ratios based on Uvigerina peregrina data from the EEP is presented in Chapter 6 and completes the foraminiferal investigations. The sediment surface data set of endobenthic species U. peregrina shows a clear correlation between water depth and d18O as well as Mg/Ca ratios, thus supporting the use of this species as a reliable temperature proxy. The reconstruction of bottom water temperatures is important to understand the complex, local hydrography. The effect of bottom water delta [CO32‐] on Mg/Ca ratios in U. peregrina is apparently small and does not influence the temperature signal. A compilation of data from the EEP and literature data results in a transfer function covering a temperature range between 5 and 20°C; the resulting function (R²= 0.77) estimates temperatures with a precision of ±1.6°C. This new transfer function provides a more precise estimation of Mg/Ca‐derived temperatures in the EEP for endobenthic species U. peregrina than previous approaches.

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