Bivalve Shells—Unique High-Resolution Archives of the Environmental Past - Towards an Interdisciplinary Approach in Earth System Science - page 163

increment width was measured using the image processing software analySIS
(Olympus, version 5.1).

2.2 State of Preservation

The shells of A. islandica consist of aragonite (CaCO 3 ), trace elements and organics
(e.g., Sch ne 2013 ). After burial and in terms of fossilisation, several factors such as

heat and pressure at depth, as well as hydrothermal

ö

fluids, can cause alterations in
the shell carbonate, e.g., recrystallization from pristine aragonite to the more stable
CaCO 3 polymorph calcite (e.g., Bathurst 1964 ). In most cases the recrystallization
process involves a dissolution and recrystallization process (neomorphism, e.g.,
Maliva 1998 ), which would replace the pristine stable oxygen isotope ratio within
the carbonate and erase the associated environmental signal in the shell (e.g.,
Hendry et al. 1995 ).
Due to its high spatial resolution of a few hundred nm CRM provides an ideal
tool for shell carbonate analysis. For our measurements on Pliocene specimen
AI-TjBe-01 we used a WITec alpha 300 R instrument, equipped with a diode laser
(excitation wavelength 532 nm) and a 20

fl

×

Zeiss objective. Details on the mea-
surements can be found in Nehrke et al. ( 2012 ).

2.3 Frequency Analysis

The growth record of shell AI-EgLo-02 was detrended using a cubic spline (JMP
software, version 9.0.1 by SAS Institute Inc. 2007), and a standardized growth index
(SGI) was calculated following Butler et al. ( 2010 ). The subsequent frequency
analysis was conducted using kSpectra software (version 3.4 by SpectraWorks) with
settings according to Ivany et al. ( 2011 ) and applying a Singular Spectrum Analysis
(SSA) and the Multi Taper Method (MTM). Furthermore, we used wavelet
transformation to examine whether quasi-periodic signals were stationary over
time ( http://ion.researchsystems.com/IONScript/wavelet/ ), following Torrence and
Compo ( 1998 ). Growth records of specimens Ai24568 and AI-TjBe-01 have not
been analysed.

18 O)

2.4 Stable Oxygen Isotopes ( δ

During shell formation, A. islandica incorporates oxygen isotopes in equilibrium
with the surrounding seawater (Weidman and Jones 1994 ). Since the incorporation
of lighter oxygen isotopes is facilitated during higher temperatures (Grossman and
Ku 1986 ),

18 O of most bivalve species provides

the oxygen isotope ratio δ