Applied methods

2.2.1 Foraminiferal counting

This study provides the first living foraminiferal abundance analyses from a multiple open/closing plankton net at the Manihiki Plateau. The recovered (coloured) material consisted of plankton from various size fractions (Figure 2.3). Plankton net samples were sieved over nets with a mesh size of 1000 µm and 63 µm to select foraminiferal tests more efficiently. As spinouse species often tend to stuck to larger organic material, the filtered material >1000 µm was exam-ined for foraminiferal tests as well. From the size fraction 125 – 1000 µm intact planktonic forami-nifera were wet picked collected using a binocular microscope, and dried afterwards. Foraminif-era with coloured cytoplasm in the early chambers were selected, which we infer represent spec-imens that were collected alive or shortly after they died. This thesis primarily focuses on forami-nifera >125 µm. This size fraction is well established in paleoceanographic research, in which many studies focuses on foraminifera in the size the range between 250 and 500 µm [Dekens et al., 2002; Wara et al., 2005; Kiefer et al., 2006; Knudson and Ravelo, 2015a; Nürnberg et al., 2015]. We enlarged the size fraction slightly to have broad overview over the even smaller sized foraminiferal species. Depending on the amount of material approximately 200 – 400 foraminifera were identified and selected, either in the whole sample or in aliquots. The planktonic foraminifer-al taxonomy follows the work of Parker [1962], Bé [1977] and Hemleben et foraminifer-al. [1989].

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Figure 2.3. Multinet sample from 0 – 50 m water depth show the various size fractions of the recov-ered material.

Several studies have shown that Globigerinoides ruber (white) exists in different morphotypes that dwell in slightly different water depths near the sea surface [e.g. Wang, 2000; Steinke et al., 2005; Kuroyanagi et al., 2008]. The determination of the morphotypes sensu strictu (s.s.) and sensu lato (s.l.) follows the concept of Wang [2000] (Figure 2.4), after which G. ruber s.s. has spherical chambers sitting symmetrically over previous sutures with high arched apertus and G ruber s.l. corresponds to more compressed subspherical chambers with a small aperture. The morphotype G. ruber s.s. has been found to dwell in shallower water depths and was thus select-ed for the analysis. Nevertheless, due to limitselect-ed amount of material, specimens of the slightly deeper dwelling G. ruber s.l. were also included in our dataset when necessary.

Figure 2.4. The different morphotypes of Globigerinoides ruber. a: morphotype sensu strictu (s.s) and b: sensu lato (s.l.) [figure modified after Wang, 2000].

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For comparisons with published data, the foraminiferal density in the water column was calcu-lated using the following formula: Alfred-Wegener-Institut, Helmholtz Zentrum für Polar- und Meeresforschung (AWI), Germany, using Finnigan MAT 251 and MAT 253 isotope mass spectrometers that are coupled to automatic carbonate preparation devices Kiel II and IV, respectively. The stable isotope ratios are given in permil (δ)-notation, calibrated via international standard NBS 19 to the Vienna PeeDee Belemnite (VPDB) scale. They are determined as follows:

δ

(

)

heavy isotope light isotope

The precision of the measurements, determined over a one-year period and based on repeated analysis of an internal laboratory standard (Solnhofen limestone), is ±0.06 ‰ and ±0.08 ‰ (1 σ) for carbon and oxygen isotopes, respectively.

The isotopic composition of seawater samples were determined on a Delta S for the δ¹⁸Oseawater and on a Gas Bench II MAS 252 for the δ¹³CDIC at the AWI. The δ¹⁸Oseawater values were calibrated to the Vienna standard mean ocean water (VSMOW) scale and δ¹³CDIC via the international standard NBS 19 to the VPDB scale. The precision based on an internal laboratory standard (Ocean 3 and DML for δ¹⁸O_seawater and Solhofen limestone for δ¹³C_DIC) measured over a one-year period is ±0.03 ‰ (1 σ) for δ¹⁸O_seawater and ±0.1 ‰ (1 σ) for δ¹³C_DIC.

2.2.3 Determination of trace element ratios

The Mg/Ca ratios of the foraminiferal shells were obtained via laser ablation coupled to a In-ductively Coupled Plasma-Mass Spectrometer (LA-ICP-MS). Compared to solution based trace element ICP-MS analyses, LA-ICP-MS requires only very little sample material and only minimal pre-treatment as surface contamination can be removed by pre-ablating samples prior to analy-sis. Further, it allows to obtain a large range of element concentrations in solid samples and to detect element variabilities within samples.

As many chambers as possible were measured to ensure to have sampled as much test ma-terial as possible (Figure 2.5). It has been shown that the large final chamber makes up the bulk

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of solution based measurements [Hemleben and Bijma, 1994] and average element ratios deter-mined over the whole tests, are in good agreement with published empirical calibrations on bulk foraminifera [Kunioka et al., 2006; Spero et al., 2015]. Thus, the laser ablation method is ideally suited for the trace element analyses of the multinet samples, which contain only little measure-able material.

The geochemical analyses were carried out with the Excimer ArF 193 nm laser ablation sys-tem from NEW Wave ESI with a two-volume ablation cell design, coupled to an Agilent 7500cs LA-ICP-MS at GEOMAR, Helmholtz Centre for Ocean Research Kiel, Germany. A more detailed description about the settings for the laser ablation analysis is given in Chapter 3.2.3.

Figure 2.5. Foraminiferal species analysed for Mg/Ca. Holes show the penetration of the laser. White line denotes always 100 µm. a: Globigerinoides ruber, b: Globigerinoides sacculifer, c: Neoglobo-quadrina dutertrei, d: Pulleniatina obliquiloculata and e: Globorotaloides hexagonus.

2.2.3.1 Mg/Ca paleothermometry

Foraminiferal Mg/Ca ratios have become an established proxy to reconstruct past climate sys-tem changes over the last decades. The uptake of Mg into biogenic calcite shows an exponential dependency on temperature after:

Mg Ca = B * exp(A *T ) (3)

with the pre-exponential and exponential constants given as B and A, respectively, and T denotes the δ¹⁸O calcification temperature [Nürnberg et al., 1996; Lea et al., 1999; Elderfield and Gansson, 2000; Dekens et al., 2002; Anand et al., 2003; Regenberg et al., 2009; Friedrich et al., 2012]. However, the Mg incorporation into foraminiferal tests is highly biologically mediated

2.0 MATERIAL AND METHODS

[Nürnberg et al., 1996; Lea et al., 1999; Dueñas-Bohórquez et al., 2009; 2011]. Due to these so-called “vital effects” species-specific differences in the uptake of Mg into the foraminiferal calcitic test occur. As a consequence, a large number of culture-based, sediment-trap and core-top stud-ies have generated many different generic and specstud-ies-specific paleotemperature equations that have basic similarities but also differ slightly from each other [Nürnberg et al., 1996; Dekens et al., 2002; Anand et al., 2003; Cléroux et al., 2008; Regenberg et al., 2009]. These small differ-ences might lead to different temperature estimates when applied to the same Mg/Ca ratio.

Therefore, different paleotemperature equations were tested (Table S3.5.3) to find the most suit-able temperature equation for the planktonic foraminifera from the multinet.