Size fraction analysis of fish-derived carbonates in shallow sub-tropical marine environments and a potentially unrecognised origin for peloidal carbonates
Introduction
Micrite and microspar are common components of carbonate successions in the geological record, and are usually considered to result from the diagenetic alteration of fine-grained skeletal or abiotically precipitated carbonates (Lasemi and Sandberg, 1984). However, in most cases clear evidence for such an origin is enigmatic at best, and thus a range of known contemporary sources of mud-grade carbonate are usually invoked to explain mud production, these including: 1) mechanical and/or biological break down of skeletal carbonates, such as calcareous green algae and calcareous seagrass epiphytes (Lowenstam and Epstein, 1957, Land, 1970, Nelsen and Ginsburg, 1986, Gischler and Zingeler, 2002, Gischler et al., 2013); 2) biogenically-induced precipitation, such as that associated with microbial activity and blooms of unicellular algae; a process that has been linked to ‘whiting’ events (Greenfield, 1963, Robbins and Blackwelder, 1992, Yates and Robbins, 1995, Yates and Robbins, 2001); and 3) abiotic precipitation, which has also been linked with ‘whiting’ events (Shinn et al., 1989, Milliman et al., 1993, Morse et al., 2003). In addition, recent studies have identified teleost (bony) fish as important contributors to global marine carbonate production (Wilson et al., 2009) and as a potentially important source of mud-grade carbonate, especially in sites of high fish biomass (Perry et al., 2011).
The physiological processes underpinning carbonate production in fish have been described in detail elsewhere (see Walsh et al., 1991, Wilson et al., 2002). In brief, marine teleosts regulate their hydration levels by absorbing ingested seawater through epithelial cells in their intestines. Prior to absorption, dissolved Ca2 + (and some Mg2 +) is removed from this seawater via precipitation of Mg calcite (along with smaller quantities of other carbonate phases); this process occurring within intestinal fluids under conditions of highly elevated CaCO3 saturation caused by secretion of metabolically-derived HCO3− into the intestine. The resulting carbonates are excreted into the open water column as generally sand-grade pellets of loosely aggregated mud-grade particles; the latter occurring with a diverse range of distinctive morphological forms (morphotypes) that vary with species (Perry et al., 2011, Salter et al., 2012).
Initial grain size measurements of these morphotypes, although not exhaustive, indicate length ranges of < 0.5 to > 50 μm, with different morphotypes occurring within reasonably narrow ranges (Perry et al., 2011, Salter et al., 2012). Given that the overall size range spans two orders of magnitude, and that differences in settling and entrainment potential must apply, it is predicted that different particle morphotypes should have different transport and depositional fates. Since the genetic origins of carbonate grains in the rock record are frequently used as indicators of palaeo-environment, palaeo-climate, and palaeo-ecology (e.g., Flügel, 2004), it is important that the potential transport and subsequent deposition of these grains is more fully understood. This paper presents an initial consideration of these issues through detailed analysis of the grain sizes of carbonates produced by a range of common Caribbean fish species; these data being used to inform a conceptual model concerning the post-excretion re-distribution of these crystals on and around shallow sub-tropical carbonate platforms, using the Bahamas as an example.
Furthermore, previous studies regarding the sedimentary significance of fish-derived carbonates have done so only with respect to the mud fraction (Perry et al., 2011, Salter et al., 2012); such studies assuming that excreted pellets, which disaggregate readily at pin prick when dry, eventually break down and release their component mud-grade particles individually. However, two alternative possibilities exist that have received no attention thus far, these being that: i) pellets remain intact after excretion; and ii) some particles released upon pellet disaggregation are strongly bound to other particles such that morphotypes are not actually released as individual particles, but rather as larger particle clusters. With regard to the former, existing descriptions of fish-derived carbonate pellets are remarkably similar to those given for numerous sedimentary pellets of presumed faecal origin from the surface sediments of shallow platform settings in the Bahamas (Illing, 1954, Ginsburg et al., 1958, Newell et al., 1959, Purdy, 1963, Reijmer et al., 2009). Such pellets have been found to occur with varying degrees of cohesive strength (Illing, 1954); a phenomenon that has been attributed to different degrees of induration. Thus, carbonate pellets initially similar to fish-derived pellets not only have the potential to be preserved intact in surface sediments, but can also undergo cementation; this process increasing their preservation potential and possibly resulting in the conversion of pellets to peloids (Rankey and Reeder, 2010). This paper therefore also investigates the possibility that a proportion of fish-derived carbonates may represent a hitherto unrecognised source of sand-sized carbonate pellets and peloids.
Section snippets
Collection and preparation of carbonates
Carbonates were collected during November 2009 and July 2010 from six fish species common to the Bahamas; species being selected to ensure the full range of crystal forms described elsewhere (Salter et al., 2012) were represented (see Table 1). Samples were collected at the Cape Eleuthera Institute, Eleuthera Island, the Bahamas (24° 50′N, 76° 20′W; Fig. 1) following protocols described by Perry et al. (2011), whereby recently captured fish were held in aquaria tanks so that excreted gut
Pellet morphologies
Pellets produced by all six fish species typically exhibit sub-spherical to ellipsoidal morphologies at the point of excretion, with sub-rounded to sub-angular grain outlines (Fig. 2); these being broadly similar among species. However, pellets produced by great barracuda are generally more elongate (to the point of being quite distinctive in the context of shallow platform sediments; Fig. 2F), and analysis of aspect ratio data (Table 2) indicates they are significantly more elongate than those
Platform-top accumulation of fish-derived carbonates
Fish-derived carbonates, in their various aggregated and disaggregated states post-excretion, have a size range spanning four orders of magnitude (0.1 to > 1000 μm in length), whilst particles released from disaggregated pellets span more than three orders of magnitude (0.1 to 177 μm in length), with particle size closely related to morphotype. This large grain size variation has clear implications for the transport potential and depositional fate of these particles, and thus for sedimentary
Conclusions
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Numerous carbonate pellets produced and excreted by marine teleost fish may remain intact and contribute to the sediment sand fraction of platform-top environments in tropical and sub-tropical regions, thus representing a hitherto unrecognised source of carbonate pellets in these settings. Post-excretion pathways are likely similar to those followed by faecal pellets, with pellets produced in quiescent settings retaining something close to their size (very fine to coarse sand) and morphology at
Acknowledgements
MAS was funded through a Ph.D studentship and NERC Grant NE/K003143/1, with additional support from an IAS postgraduate grant. CTP and RWW were funded through NERC Grants NE/K003143/1, NE/G010617/1 and NE/H010092/1 and BBSRC Grants BB/D005108/1 and BB/F009364/1. The authors are grateful to Annabelle Brooks, Aaron Shultz, Tyler Sclodnick, and Thiago Soligo at CEI for assistance with fieldwork.
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