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doi:10.2204/iodp.proc.320321.218.2014

Introduction

The eastern equatorial region of the Pacific Ocean is an important component of the global climate system and is crucial for understanding climate evolution (Lyle et al., 2008). The Integrated Ocean Drilling Program (IODP) Expedition 320/321 “Pacific Equatorial Age Transect” (PEAT) program, drilled using the R/V JOIDES Resolution, aimed to work toward continuous high-resolution composite records of environmental change in the equatorial region between 56 Ma and the present (Lyle et al., 2010) with the top part of Site U1338 aiming at recovering late Miocene to early Pliocene sediments (Fig. F1) (see the “Expedition 320/321 summary” [Pälike et al., 2010]). The recovered sediments are carbonate rich with abundant coccolithophores and foraminifers (see the “Site U1338” chapter [Expedition 320/321 Scientists, 2010]), which all constitute excellent archives for various geochemical proxies. However, in order to make robust inferences based on geochemical records from these calcareous fossils, good preservation of the foraminiferal and coccolithophore calcite is essential. Here, we evaluate the preservation of specific benthic and planktonic foraminifer species and coccolithophores in fine fraction sediment at Site U1338 (2°30.469′N, 117°58.178′W; 4200 m water depth).

For geochemical applications using foraminiferal calcite, it is crucial that the calcite test is well preserved. Pristine benthic and planktonic foraminifer tests consist of calcitic microgranules (~0.1 µm diameter) that are laterally fused (Pearson and Burgess, 2008; Sexton and Wilson, 2009). The outer and inner test walls of well-preserved foraminifers are smooth on a submicrometer scale, and cross sections of the test walls have a pervasive microgranular structure (Sexton et al., 2006). The average chemical composition of the tests can be affected by later recrystallization or overgrowth with calcite that was precipitated from a secondary water mass with a different temperature and chemical composition than the water mass in which the foraminifers lived (Pearson et al., 2001; Sexton et al., 2006). Dissolution can also remove primary calcite, thereby altering the average chemical composition of the foraminifer test (Edgar et al., 2013).

Planktonic foraminifer test preservation has been described as gradationally varying between pristine “glassy” and less well preserved “frosty” preservation (Sexton et al., 2006). Glassy planktonic foraminifers are generally found in clay-rich sediments, where permeability is lower, thereby minimizing the interaction between pore waters and the primary calcite and reducing the chance of recrystallization (Pearson et al., 2001). The glassy specimens, which appear translucent under a reflected light microscope (RLM), retain their smooth test walls and the original microstructure (Sexton et al., 2006). Frosty planktonic foraminifers appear opaque when viewed under RLM and are often recovered from biogenic oozes and chalky deep-sea deposits (Pearson and Burgess, 2008). With increasing opaqueness, frosty specimens frequently show increasing overgrowth on the interior and exterior test walls, and the test walls show widespread recrystallization of the microgranular calcite into large inorganic calcite crystals (Pearson and Burgess, 2008; Sexton et al., 2006). Widespread recrystallization and dissolution also causes the foraminifer to become mechanically weaker (Pearson and Burgess, 2008), thereby increasing the chance of fragmentation during deposition or sample processing. In addition to diagenetic alteration of the primary foraminiferal calcite, a further concern is when planktonic foraminifer test chambers contain calcite “infilling” of a different origin, such as coccolithophore calcite (Sexton et al., 2006).

As planktonic foraminifers undergo diagenetic alteration on the seafloor, recrystallization and overgrowth with inorganic calcite causes an increase in the δ¹⁸O values of the foraminiferal calcite (Pearson et al., 2001). If used for temperature reconstructions, recrystallized frosty planktonic foraminiferal calcite δ¹⁸O should provide lower temperature estimates than pristine glassy planktonic foraminiferal calcite δ¹⁸O (Sexton et al., 2006). No significant offset was observed between glassy and frosty δ¹³C values (Sexton et al., 2006).

Benthic foraminifers are considered less receptive to diagenetic processes such as dissolution, recrystallization, and overgrowth (Edgar et al., 2013; Sexton and Wilson, 2009). This is predominantly because benthic foraminiferal tests are generally more heavily calcified than those of planktonic foraminifers, and are therefore thought to be more resistant to both dissolution and recrystallization (Sexton and Wilson, 2009). However, recent high-resolution scanning electron microscopy (SEM) studies of benthic foraminifers have shown that the benthic species show similar levels of recrystallization at micrometer and submicrometer scale as previously found in planktonic species (Edgar et al., 2013; Sexton and Wilson, 2009). In addition, badly preserved samples show evidence of overgrowth with large inorganic calcite crystals on the internal and sometimes external test walls (Sexton and Wilson, 2009). Distinguishing the degree of internal recrystallization on the basis of external test wall preservation alone is problematic, as Edgar et al. (2013) showed that the preservation of the microgranular structure was variable between various frosty specimens, even when the outer test microstructure of the different specimens was similar. Despite similar levels of submicrometer recrystallization occurring in benthic and planktonic foraminifers, diagenetic alteration does not seem to create any offset in benthic foraminifer δ¹⁸O and δ¹³C values compared to δ¹⁸O and δ¹³C from pristine specimens (Edgar et al., 2013; Sexton and Wilson, 2009). Edgar et al. (2013) suggest that the absence of an offset is related to recrystallization of the benthic foraminiferal calcite, most likely occurring in pore waters of similar temperature and chemical composition to the bottom waters where the original benthic foraminiferal calcite precipitated.

Coccolith calcite is generally preferentially preserved compared to planktonic foraminiferal calcite, as coccolith calcite is less receptive to dissolution and recrystallization (Bown and Dunkley Jones, 2012; Schmidt et al., 2006; Young et al., 2005). The preferential preservation of coccoliths is predominantly due to a difference in calcite crystal size (Adelseck et al., 1973; Hover et al., 2001). First, smaller crystals are more susceptible to dissolution than larger crystals (Hover et al., 2001). Coccolithophores precipitate the coccoliths from an internal vesicle and form two main types of coccoliths: holococcoliths and heterococcoliths (Young et al., 2005). Holococcoliths are made up of 0.1 µm euhedral calcite crystals and are therefore very susceptible to dissolution (Bown et al., 2008). However, heterococcoliths are made up of larger, more complexly shaped, single crystals of calcite (Schmidt et al., 2006), which makes them far less susceptible to dissolution (Bown et al., 2008). Holococcoliths are rarely preserved outside of clay-rich sediments (Bown et al., 2008). Dissolution (or etching) of heterococcoliths can range from slight etching (affects delicate structures; creates a serrated edge of coccolith), moderate (delicate structures destroyed; causes coccolith outlines to become irregular) to strong (delicate taxa are rare; coccolith fragments are abundant) etching (Blechschmidt, 1979; Roth and Thierstein, 1972; Roth, 1973, 1983; Su et al., 2000). Recrystallization of coccolith calcite is not common, as holococcoliths mostly dissolve, and the large crystal size of the heterococcoliths makes overgrowth more thermodynamically favorable (Adelseck et al., 1973; Hover et al., 2001). Overgrowth of heterococcoliths can also range from slight (some irregular secondary growth; slight thickening of central coccolith areas) to moderate (common irregular secondary growth; delicate structures overgrown and difficult to recognize) to strong (overgrowth hinders identification) (Adelseck et al., 1973; Roth and Thierstein, 1972; Roth, 1983; Su et al., 2000).

In this data report, foraminifer preservation is assessed using backscattered electron (BSE) SEM on multiple specimens of Cibicidoides mundulus and Globigerinoides sacculifer. Coccolithophore preservation is assessed using BSE and secondary electron (SE) SEM on the sediment fine fraction (<63 µm). The main components of the fine fraction sediment are qualitatively evaluated using energy dispersive spectroscopy (EDS) on an SEM.

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