Halimeda Bioherms

Introduction

Halimeda Bioherms – two words with separate biological and geological connotations – have been used cojoined since 1985 by Davies and Marshall (1985) and Orme (1985) to describe Holocene mounds (bioherms) of dominantly Halimeda derived carbonates in water depths generally deeper than 30 m (sometimes 20 m) and forming undulating banks. Some confusion does exist within the literature, not because colleagues do not understand the use of the above authors but because of other discipline-related terms coined and used both before and since 1985. The geological understanding of Bioherm, therefore, needs to be clarified, as does its relations to a term coined at the same time – Biostrome – and their likely (possible?) relation to the more recently coined biological term, Meadows. For these reasons, it is proposed to very briefly clarify as follows:

First, Halimeda are green calcareous precipitating algae from the phylum Chlorophyta in the order Bryopsidales and in two families – Halimedaceae and Udoteaceae. The taxonomy is described by Hillis-Colinvaux (1980) and in the Great Barrier Reef in various papers by Drew and Abel (see references in Drew and Abel, 1988a, b). There is no confusion over the term Halimeda, only sometimes over the identification of species.

Secondly, Bioherm is a geological term coined by Cummings (1932) to describe both bedded and nonbedded lens-like or mound-like accumulations, comprised of the often in situ accumulation of invertebrate organisms. The original definition had reefs in mind, invoking topographic relief above the sea floor, an in situ framework and rapid accumulation. In a modern sense, the bioherms described in this entry are not reefal, yet they are mound-like, often with significant relief above the surrounding sea floor and with internal characters, which are bedded or nonbedded and composed of in situ and derived biologic accumulations that have accreted rapidly. In a geological sense, mound-like structures sometimes show substantial mud and few framework organisms (many Carboniferous mounds) and are still legitimately termed bioherms. In the original definition, Cummings (op cit) distinguished bioherms from related structures termed Biostromes (see review by Kershaw, 1994), defined as nonreefal flat bedded structures often but not necessarily comprised of organic accumulations. It is noted here because Orme (1985) and Orme et al. (1978) rightly pointed out that in the northern Great Barrier Reef, large areas of the mid shelf behind the Ribbon Reefs (termed “back-reef” by Orme op cit) appear to be comprised of crudely bedded laterally extensive nonmounded “biostromal sediments” and are associated with Halimeda Bioherms (called “banks” by Orme et al., 1978). Later, Drew and Abel (1988a) described the surface characters of such areas as “Meadows” and Hillis-Colinvaux (1988), in a reef lagoonal environment, postulated that Meadows might evolve into Bioherms.

In the present contribution, Halimeda Bioherms are used to denote mounds showing relief above the surrounding sea floor and built by the in situ accumulation of Halimeda together with other infauna. They are sometimes called Banks (Orme and Salama, 1988) although Banks should rightly be reserved for much larger structures, for example, Bahama Banks. In the Great Barrier Reef and elsewhere, Halimeda Bioherms are associated with often larger areas having an extensive sheet-like form and called Biostromal areas (Orme and Salama, 1988, p. 136), the surface of such areas are called Halimeda Meadows by Drew and Abel (1988b). Thus, the origin of biostromes (a 3-D geologic term) might be elucidated by studying their surface “Meadows” (a 2-D biological term). Further, the origin of the Bioherms might be determined through studies of the spatially related Meadows/Biostromes. This entry follows this treatment. First however, the biology of Halimeda Bioherms and their related meadows are briefly reviewed.

The biology of Halimeda bioherms and adjacent meadows (biostromes)

Halimeda are a green alga that belongs to the phyllum Chlorophyta in the order Bryopsidales. Halimeda are described by Hillis-Colinvaux (1980, 1988) as being either Rhipsalian and possessing a holdfast or non-Rhipsalian and possessing none or a limited holdfast. In the modern environment, Halimeda occur as an accessory component of reefs, or as principal components of bioherms and meadows.

Halimeda Bioherms generally occur above a base line of 50 m water depth while meadows generally occur below this depth. The most extensive publications on their biology are those of Drew and Abel (1988a, b). The species composition of bioherms and meadows in the Great Barrier Reef is shown in Table 1.

Halimeda Bioherms. Table 1 The species diversity of Halimeda meadows compared to adjacent reefs in the Great Barrier Reef region

Species diversity is high. A total of 12 Halimeda species, 2 Udotea and 1 Penicillus occur in water depths above 50 m dominated by the non-rhipsalian H. opuntia and H. hederacea while in water depths greater than 50 m, only H. copiosa and H. hederacea occur. Growth occurs via multiple obscure holdfasts restricted to the surface of the sediments. In other areas of the Pacific and Indian Oceans where Halimeda bioherms and meadows have been described, the same species predominate. Accumulations of Halimeda as meadows have also been reported from the lagoon of Enewetak atoll (Hillis-Colinvaux, 1988) where the dominant species are the rhipsalian H. incrussata and H. cylindrica.

The global distribution of Halimeda bioherms

The Great Barrier Reef

Halimeda Bioherms have been described in two areas: (1) two parts of the northern GBR and (2) in the Swains region of the southern GBR (Figure 1).

Halimeda Bioherms. Figure 1

Distribution of Halimeda bioherms in the Great Barrier Reef. They occur in three areas, Northern reef region, Lizard Island to Cooktown, and in the Swains reefs region.

In the northern Region, Halimeda Bioherms have been described by two separate groups in the Lizard Island – Cape Flattery region and in the Ribbon 2–Ribbon 7 region.

The Lizard Island – Cape Flattery region (Figure 1) has been described by Orme and coworkers (Orme et al., 1978; Orme, 1985; Orme and Salama, 1988). They report that 26% of the total shelf area (effectively the outer shelf area behind the shelf edge reefs) between latitudes 14.27S and 15.02S is occupied by Halimeda litho-facies (Bioherms and Biostromal meadows). Halimeda bioherms are up to 19 m thick and sit on a prominent seismic reflector assumed to be the transgressed pre-Holocene/Pleistocene unconformity. The bioherms (= banks in Orme and Salama, 1988) that rise to within 25 m of current sea level are best developed in the eastern part of the region and their surface is mounded and “cloaked by Halimeda meadows – and a limited veneer of reef coral – especially in troughs.” Little sedimentologic data on the Halimeda sediments and no dating have been published.

The bathymetry, seismic structure, submersible examination and photography, sedimentology, and biology of Halimeda banks in the region east of Cooktown (Figure 1) have been described by Davies and coworkers (Davies and Marshall, 1985; Phipps, Davies and Hopley, 1985; Marshall and Davies, 1988). They report that between 15.10S and 15.35S in the lee of Ribbons 2–7, the “reefless” tract supports a luxurious growth of Halimeda that during the Holocene have developed Halimeda bioherms in water depths between 30 and 50 m.

In the southern Great Barrier Reef region, Halimeda bioherms have been reported from the Swains reefs region, west of Frigate Shoals in water depths of 20–32 m (Figure 1) (Searle and Flood, 1988).

The Sahul shelf

On the Sahul Shoals, Halimeda are integral components of large “banks” such as Big Bank, Snow White, Happy, Grumpy and Udang (Rees et al., 2007). The banks are generally flat topped with few bioherms in the sense of those described from the Great Barrier Reef.

The Java Sea bioherms

In the Java Sea, Phipps and Roberts (1988) describe Halimeda bioherms on K Bank in the eastern part of the Makassar Straits (Figure 3). They are similar to those described from the Great Barrier Reef, if not a little larger.

Halimeda Bioherms. Figure 3

Halimeda bioherms in the Java Sea, in the region of the Macassar Strait (After Phipps and Roberts, 1988).

The Miskita bank/channel bioherms

Several generations of Halimeda bioherms occur on the shelf northeast of Nicaragua (Figure 4), as seen mainly on multichannel seismic sections.

Halimeda Bioherms. Figure 4

Halimeda bioherms on the Nicaraguan shelf in the region of the Miskita Channel (After Hine et al., 1988).

Descriptive features of Halimeda bioherms

In the Great Barrier Reef, Halimeda bioherms occur over large areas behind the outer reefs, as “fields” or “complexes” of inter-fingering or inter-merging individual elongate bioherms, 150 m long (N–S) and 100 m wide (W–E), slightly flat topped and sloping away in all directions at 5–15° (Figure 5a). The most striking surface feature is a forest of soft green algae, sometimes 50 cm thick on the tops and thinning down the sides of each bioherm. This forest is dominated by various species of Caulerpa and Halimeda. Structurally, it is similar to rain forest in that there is a thick undergrowth on a sandy/gravely surface, a middle layer of less dense cover and a top canopy. This forest is an ephemeral feature, appearing and disappearing within months. Mollusks, foraminifera, and bryozoans form integral and important parts of the epifauna with Halimeda. These collectively mantle the surface of the top and sides of the bioherms but stop suddenly 3–5 m above the troughs between adjacent bioherms.

Halimeda Bioherms. Figure 5

(a) West to east seismic section across the outer Cooktown shelf showing bedded nature of the Halimeda banks in 30 m of water. (b + c) Uninterpreted and interpreted seismic section across the banks on the outer Cooktown shelf. The multigenerational nature of the banks together with their planal Pleistocene growth surface clearly visible. (d) Seismic section of thick Halimeda banks in the Makassar Strait, Java Sea. The banks are clearly sitting on a planar surface interpreted as the Pleistocene.

Seismic data shows in the Great Barrier Reef and the Java Sea, the bioherms are up to 15 m thick in the GBR (Figure 5a, b + c) but are substantially thicker (+20 m) in the Makassar examples (Figure 5d). In the northern Great Barrier Reef, the bioherms sit on a prominent seismic reflector (Figure 5a + c) which dips from around 33 m immediately west of the reefs to around 65 m some 5–10 km west of the reefs. The bioherms themselves also decrease in size and increase in depth in the same direction. The reflector in Figure 5a + c, is a leached skeletal, limestone whose vugs are partially filled with soil. The geometry of the bioherm complexes bear no relation to the geometry of the prominent reflector. A similar seismic reflector also underpins the bioherms in the Makassar Straits (Figure 5d).

Bedding is the commonest internal structural feature (Figure 5a–d); internally bioherms are sometimes bedded, sometimes transparent and often multigenerational. Individual bioherm complexes show bedding suggestive of several generations of growth. Tops are frequently seismically transparent. Sometimes, there are two seismic facies – a lower indistinctly bedded unit that merges laterally with a biostromal sheet facies and an upper bedded facies forming the upper two thirds of the banks.

Sedimentologically, as seen in cores (Figure 6), the bioherms are comprised of Halimeda sands and gravels in an olive green matrix. Large foraminifera, Marginopora vertebralis and Alveoinella quoyi are subordinate but important parts of the gravel fraction, as are mollusks and bryozoans. The Halimeda are dominantly H. opuntia, v. hederacea, and H. copiosa with subordinate amounts of H. fragilis, H. discoidea, and H. gracilis. Texturally the bioherms are gravelly, sandy muds throughout but with slight textural variations down the core. At the surface, the gravel fraction (largely Halimeda) comprises 50%, this dropping to 20–30% within 1 m of the top. Deeper in the core the gravel fraction either remains at 20–30% or drops even further to less than 10%. The sand fraction in most cores stands at 20–30% and is comprised dominantly of broken Halimeda leaves with contributions also from foraminifera, bryozoans, and mollusks. Mud in the cores measures around 10% in the top half meter of the cores and then increases substantially to between 30 and 50% throughout the rest of the core. Approximately half of this mud fraction is carbonate and half non-carbonate or terrigenous (quartz, kaolinite, and smectite). However, in the Swains Reefs bioherms, the mud matrix is totally carbonate and devoid of terrigenous material, reflecting the distance of these bioherms from the Australian mainland. In the Java Sea, Phipps and Roberts (1988) describe the bioherms as disarticulated Halimeda plates with a fine grained matrix of foram-rich carbonate mud (usually less than 40%), only a small percentage of which is non-carbonate (volcanic shards and siliceous spicules). In some cores, Halimeda plates form a disorientated open-textured accumulation. Locally, within cores, sediments are composed of coarse Halimeda plates, some of which are still unbroken. Occasionally, layers 5–10 mm thick are comprised of Halimeda plates orientated parallel to the bioherm surface.

Halimeda Bioherms. Figure 6

Cores from the Halimeda banks on the Cooktown shelf. The positions of detailed photographs are shown on the cores.

Rates of growth of Halimeda bioherms

Biologic studies report a wide range of growth rates for Halimeda, varying from 420 g CaCO₃ m² year⁻¹ in the Florida Straits to 2,234–3,000 g m² year⁻¹ in the Great Barrier Reef (Drew, 1986). The high rates in the GBR are attained by the plant doubling the biomass of the colony every 15 days. This represents an enormous production of carbonate sediments indicating that Halimeda are a major contributor to tropical marine environments. The bioherms described in this entry are testimony to that conclusion. Radiocarbon dating (Table 2) provides some indication of the vertical accumulation rates of the bioherms in the Great Barrier Reef and Makassar Straits.

Halimeda Bioherms. Table 2 Vertical accumulation rates of bioherms in the Great Barrier Reef and Makassar Straits

Table 2 indicates clearly that accumulation rates have been similar in both the Great Barrier Reef and in the Makassar Straits. Depending on porosity, accumulation rates of close to 3 m/1,000 years are very close to Drew’s estimates and confirm that not only are Halimeda significant contributors to reef ecosystems, but that they are capable of producing geologically significant ecosystems in their own right.

Relations between bioherms (banks) and meadows (biostromes) and the lagoonal meadows at Enewetak

In the Great Barrier Reef, Bioherms are intimately associated with widespread crudely bedded areas which are not mounded and which have been described by Drew and Abel (1988a) as Meadows. Such areas occupy a range of water depths but often deeper than 50 m and extending down to 96 m in places. Similar deposits are reported to occur extensively over K bank east of Makassar Straits. The growth of such features and their relations to the truly biohermal deposits is, therefore, pertinent to the current discussion. Clues as to growth were defined at Enewetak by Hillis-Colinvaux (1988) where various ages of thalli (successional development) were observed to spread across unconsolidated sediment producing a raised terrace above neighboring barren sands. First, an extensive holdfast system developed together with buried thalli; secondly, Halimeda plates were shed and they and associated sediments were stabilized by cyanobacterial algal mats; thirdly, elevation of thalli above the sediment surface anchored by the holdfast system with further in situ accumulation of plates and platelets and further stabilization by mats; fourthly, the development of an associated flora and fauna, particularly sponges and fifthly, a thickness was achieved by continuous shedding from perennial growth and algal mat development. This may define a mechanism of growth applicable to the early stages of meadow development of the transgressed Pleistocene surface. However, Drew and Abel (1988a) showed that shallow meadows in the Great Barrier Reef are comprised of non-Rhipsalian algae without strong holdfasts. Further work is required to determine whether such meadows grow out of meadows like those at Enewetak.

Growth of Halimeda bioherms

A number of features are pertinent to growth and origin:

• 1. They grow from a prominent surface defined in the Great Barrier Reef as the transgressed Pleistocene surface. A similar reflector also occurs below the Makassar Strait bioherms and is also interpreted as the Pleistocene transgressed surface. The shape of biohermal mounds bears no relation to any minor relief features on this transgressed surface. There is little antecedence effect.

• 2. They occur intimately associated with sheet-like bedded biostromal deposits (Meadows).

• 3. In the Great Barrier Reef they occur in the lee-shelter of the Holocene outer barrier.

• 4. At least in the Cooktown region, they are largest close to the outer barrier, becoming smaller to the west.

• 5. They are comprised of in situ and derived accumulations of Halimeda leaves and an associated fauna of foraminifers, bryozoans, and mollusks, which form a series of multigenerational mounds, sometimes bedded, sometimes transparent.

• 6. In the Great Barrier Reef and in the Makassar Strait, their presence on the same Pleistocene surface that underlies the modern reefs, places their age as post Pleistocene, growing at the same time as the Reefs. Radiocarbon dating corroborates this conclusion.

The relations (if any) of Bioherms to the Biostromal meadows has intrigued many. Orme et al. (1985, 1988) and Davies and co-workers (1985, 1988) have drawn attention to the co-occurrence of bioherms and biostromes (meadows) and Davies and coworkers in particular cite a variety of evidence (seismic, bathymetric, and visual observations from submersible) to indicate that meadows grade into biohermal areas. In the Great Barrier Reef, a case can be made, therefore, to suggest that meadows (biostromes) first occupied the transgressed Pleistocene surface and evolved into bioherms in places favorable to vertical growth. However, the development of a vertical growth potential demands special conditions and Davies et al. (op cit) and Drew and Abel (1988b) point to the likely importance of enhanced nutrients as an essential requirement. Thompson and Wolanski (1984) proposed that strong tidal currents operating in the inter-reef channels produce a Bernoulli Effect, lifting off-reef water onto the shelf through passes deeper than 50 m. Further Wolanski et al. (1988) propose that off the Cooktown region, upwelling and development of a tidal jet is occurring and carrying nutrient rich water from a reef passage to the Halimeda meadows. Such upwelling is limited to channels shallower than 45 m. These mechanisms add substantial weight to the timing and modes of origin defined by Davies and Marshall (1985) and summarized in Figure 7.

Halimeda Bioherms. Figure 7

Origin of Halimeda bioherms as proposed by Davies and Marshall (1985). The key to growth are strong nutrient eddies produced by jets of upwelled oceanic waters flooding through the inter-reef passes before the modern reefs began in the time frame 11–9,000 ka (a + b). Halimeda banks grew well in this time frame. However, when the Pleistocene substrate was topped by the rising sea level (c) nutrient concentrations were not delivered specifically to the banks but, generally, to the leeward environments. Bank growth may have slowed down at this point. However, when the Holocene reefs reached sea level, flow into the back reef was again forced through the passes, leading to more nutrient availability and a spurt in the growth of more banks (d). Thus multigenerational growth relates to nutrient flux.

Significance of Halimeda bioherms

Originally the term “bioherm” was coined and used by petroleum geologists, particularly in an exploration sense, to describe features that did or could contain petroleum. Thereafter, it became synonymous with “reef-like” structures and important targets in petroleum exploration. Davies et al. (1988) described Halimeda bioherms as potential exploration targets in the Australian region. More recently, however, Davies and Marshall (1985) pointed out a different and perhaps more significant feature of Halimeda bioherms in the Great Barrier Reef at least, that is, they produce 3 kg CaCO₃ m² year⁻¹ compared to 3–4 kg for the reefs; however, the bioherms occupy a larger area. They may, therefore, lock up a greater proportion of CaCO₃ than do the reefs. Since the correlation between carbon dioxide (CO₂) levels and global temperatures was established in the ice core records, this conclusion assumes an even greater significance because it indicates not only the relevance of calcareous algae as important additional CO₂ sinks but when estimated on a global basis it also doubles the contribution of shallow water carbonate sediments (in conjunction with reefs) as CO₂ reservoirs (Rees et al., 2007).

Summary

Halimeda bioherms have been described from three parts of the Great Barrier Reef: from the Sahul Shelf, from the Makassar Straits, and from the Nicaraguan Bank. They form mounds that are up to 20 m height and are comprised of the skeletal remains of various species of Halimeda (H. opuntia, v. hederacea, and H. copiosa with subordinate amounts of H. fragilis, H. discoidea, and H. gracilis) together with large benthic foraminifera, mollusks, and bryozoans. The bioherms are bedded, sometimes transparent and often multigenerational, and sit astride the late Pleistocene transgressed surface. They have grown at the same time as the adjacent reef ecosystem and at rates almost equivalent to the reef systems. This suggests not only a critical relevance of calcareous algae as important additional CO₂ sinks but when estimated on a global basis it also doubles the contribution of shallow water sediments (in conjunction with reefs) as CO₂ reservoirs.

Halimeda Bioherms. Figure 2

Halimeda bioherms in the region of the Sahul Shoals, Timor Sea.

Cross-references

Bioherms and Biostromes

Great Barrier Reef: Origin, Evolution, and Modern Development

Halimeda

Solution Unconformities