Origin and alteration of Halimeda segments in the sediment

Halimeda species of the lineages Opuntia and Micronesicae that exhibit small segments (<

1.6 cm; high pIUS / sIUS ratio; high CaCO3 / organic ratio; comparably small central utricles) are the source of coarse Halimeda sediments (Hillis 2001; Verbruggen and Kooistra 2004).

Larger segments (> ~2 cm) of weaker calcified species (e.g., from Halimeda lineages Halimeda and Rhipsalis) breack down to needle-sized grains and are a source of aragonite needle muds (Neumann and Land 1975; Drew 1983; Multer 1988; Macintyre and Reid 1992;

Verbruggen and Kooistra 2004). Heavily fragmented segments and needles cannot be traced back precisely to communities of calcifying macro-algae. Their source of origin (e.g., algal bioherms or communities vs. abiotic precipitation) thus is difficult to determine (Macintyre and Reid 1992; Milliman et al. 1993). Furthermore, as detailed analyses of carbonate needle mud are rarely undertaken, the total carbonate sediment contribution of the genus Halimeda is largely unknown. Overall contribution to tropical carbonate sediments that originate from the genus Halimeda (i.e., needle mud and preserved segments combined) may be underestimated.

Complete and heavily calcified Halimeda segments that are found in reef sediments become altered in processes such as physical abrasion, micro-bioerosion, dissolution and secondary cementation (Perry 1998, 2000; Perry and Taylor 2006). Dissolution and cementation are predominantly controlled by the CaCO3 saturation of the sea and pore-water of the vicinal microenvironment that is also influenced by bacterial and micro-algal overgrowth, whereas physical abrasion and endolithic micro-bioerosion are concurring processes depending whether the segment is transported or deposited.

5.1.2.1 Cementation and dissolution as in-situ processes

In CaCO3 supersaturated seawater (Ωarag > 4) of tropical shallow seas the process of cementation is rapid (e.g., Friedman 1998; Grammer et al. 1999). Secondary cementation of open spaces in dropped Halimeda segments starts at the segment rim and subsequently proceeds to the inner core part. Firstly the peripheral primary utricles (pU) and secondary utricles (sU) as well as open areas in the pIUS in vicinity to the segment rim become filled with CaCO3. Cementation advances to the central parts of the segment and, gradually in-fills tertiary utricles (tU), medullary utricles (mU) and the innermost sIUS. The “ideal” end-stage of the cementation process is an entirely cemented Halimeda segment whereby the IUS and utricles become completely filled (e.g., Appendix X.II Fig. A10g). Contrary to the CaCO3

polymorph aragonite that characterizes the skeleton of a living segment, secondary cementation occasionally may consist of other CaCO3 polymorphs such as (low and high) Mg-calcite (Alexandersson and Milliman 1981; Roberts et al. 1988; Reid and Macintyre 1998). However, depending on environmental parameters, such as the CaCO3 saturation of the sea and pore-water, the rapidity and extent of secondary cementation is limited. Physical abrasion (i.e., transport-derived erosion) or micro-bioerosion and other biotically-induced processes, e.g. CaCO3 dissolution and precipitation processes caused by epibionts and endoliths, act in parallel on the segment and may further alter its external habit and internal CaCO3 microstructure (Roberts et al. 1988; Perry 1998; 2000; Nothdurft et al. 2007; Reyes-Nivia et al. 2014). However, surface dissolution and intra-granular cementation may not exclude each other as both were observed in some of the specimens analyzed (e.g., Appendix X.II Figs. A1-14). This is possible when these processes occur at different time scales and are caused by different (abiotic and biotic) processes. Dissolution mainly affects the peripheral porous primary utricle structure and the pIUS and is likely not persistent in CaCO3

oversaturated tropical waters. Well-calcified, primary cemented segments may withstand temporary dissolution longer than lesser-calcified ones. Additionally, they may gain intra-granular density over time (Alexandersson and Milliman 1981; Perry and Taylor 2006).

5.1.2.2 Physical surface abrasion during sediment transport

Physical abrasion of the segment surface structure is a common process and may be interpreted as an indication for the transport of a segment (Perry 2000). Abrasion starts at peripheral parts of the segment with removal of the primary utricles and the pIUS. Heavily abraded segments are reduced to their central parts, i.e. central medullary utricles and sIUS, by loosing the typical “butterfly” or “fan-like” segment shape (i.e., common sun-leaf segment morphotype of H. opuntia), which is altered into a more roundish pebble-like grain shape.

This also allows separation of the process from surface dissolution (i.e., chemical abrasion), which may not alter the grain shape of Halimeda segments in such a specific way (Perry 2000; Perry and Taylor 2006). During transport the physical abrasive force does not act with the same strength on the whole surface area of the segment. The outer elongated parts become abraded faster than the central parts. It is safe to assume, that this distinct abrasion pattern is induced by a rolling transport of the segment over the seafloor, presumably polished in-between other grains, which forms the observed roundish pebble-like grain shape of extensively altered segments. Finally the inner core part of a segment is preserved. This succession is in close dependency to the process of secondary cementation. Without cementation, most segments likely disintegrate before becoming a roundish grain. Particularly that may happen to transported segments of Halimeda species and segment morphotypes with wider central utricles (sU, tU and mU) and narrow IUS, lower CaCO3 / organic ratio, and thus with a low grade of lifetime primary cementation (i.e., MAC content and density).

5.1.2.3 Micro-bioerosion

Although micro-bioerosion is frequently observed within living segments, its subsequent occurrence in a sedimentary segment may largely depend on whether it was transported or long-term deposited. Physical surface abrasion during transportation will remove signs of

“lifetime” bioerosion, as it particularly affects the outer MAC rim (pIUS) where “lifetime”

bioerosion primarily takes place (Perry 1998). During phases of transportation, it is safe to assume that segments rarely become bioeroded, as constant relocation may hardly allow settlement of micro-bioeroders. As some heavily cemented and thus presumably very old segments show considerable deep intra-granular micro-bioerosion, the observation of both may indicate long-term deposition. Thus together with the segments surface characteristics and its grade of secondary cementation, the occurrence of deep intra-granular micro-bioerosion may argue for an interpretation as final and long-term sediment deposition.