Characeae-derived carbonate deposits in Lake Ganau, Kurdistan Region, Iraq
Characeae, a family of calcifying green algae, are common in carbonate-rich freshwaters. The southwestern shoreline of Lake Ganau (Kurdistan Region, northeastern Iraq) harbors dense and thick mats of these algae (genus Chara). On the lake bottom and along the shore, carbonate sands and rocks rich in the remains of stems, branches, nodes, and whorls of Chara are deposited. These deposits show all stages of growth and degradation of characean algae, including replacement and lithification into limestone. The replacement of the fragments by fine-grained calcite preserved delicate microstructures of Chara, such as cortical walls, cell shape, inner and outer layers of the stems, and reproductive organs. Based on roundness, sorting, the degree of lithification, and preserved microstructures of the grains (fragments), three facies were recognized. The first is represented by a newly formed lime sand facies showing elongated grains, poor sorting, and reduced roundness, with pristine preservation of characean surface microstructures. The second is a weathered lime sand facies, which shows better sorting and good roundness, whereas internal structures of characean fragments are still well preserved. The third is comprised of a lithified lime sand facies (grainstone), with very well sorted and rounded grains, and poorly preserved external and internal structures of the characeans. As compared to the newly formed lime sand facies, the grainstone facies shows an increase in grain size by more than 30 %, owing to precipitation of micritic lamina of possible microbial origin. Eventually, the Characeae-derived lime sands are lithified into oolitic limestones with sparry calcite cement, forming a grainstone microfacies. The present study has important implications for the interpretation of pre-Quaternary environments, as it records all stages of the fossilization process of characean green algae and highlights the role of these algae in the formation of oolitic carbonate rocks.
KeywordsCharaceae Chara Ooids Green algae Carbonate sand Freshwater carbonates Facies
Charophytes are a group of carbonate-precipitating, macroscopic green algae that forms an important floral element in the littoral zones of carbonate-rich freshwater or brackish lakes up to ~12 m deep (Tucker and Wright 1990; Platt and Wright 1991; Garcia 1994; Garcia and Chivas 2006; Détriché et al. 2008). Their stems may reach 2 m in length and consist of elongated cortical cells twisted around a long central cell (Brasier 1980; Ghazala et al. 2004). The stems contain whorled branches along their lengths that can be calcified through extra-cellular coating (Anadón et al. 2000). Therefore, these algae fossilize easily through the biomineralization of the protective covering of the reproductive organs and stems by calcium carbonate (Martin-Closas and Dieguez 1998; Anadón et al. 2000). Charophytes include the extant family Characeae, with the most common genus being Chara, and several extinct fossil families. Fossil remains of charophytes first appeared in calcareous shales of Late Silurian age (Martin-Closas 2003). Since then, the group has undergone diverse morphological changes, and their evolutionary record experienced multiple extinctions and originations. Hence, charophytes represent a reliable group of fossils in biostratigraphy and, due to their specific habitat requirements, in paleoecological reconstructions (Garcia and Chivas 2006). It is suggested that in the absence of other appropriate fossil records, extensive use of charophytes should be made in studies of continental and marginal marine sediments (Kumar and Oliver 1984). For instance, Climent-Domènech et al. (2009) reported thalli (stems and nodes) and reproduction organs of the genus Clavatoraxis in the Barremian of the Maestrat Basin (Eastern Iberian Chain), and developed a paleoecological model of this non-marine environment based on charophyte remains.
Charophyte remains typically consist of vegetative (thallus) fragments and gyrogonites (calcified fructifications), which reliably reflect the nature and quality of the water in which the plants grew (Soulié-Märsche 1991). Photosynthetic activity of charophytes leads to precipitation of autochthonous carbonates that may substantially contribute to lacustrine sedimentation (Apolinarska et al. 2011). Soulié-Märsche et al. (2010) reported on lake carbonates (“characeite”) from the Middle Atlas Mountains, Morocco, which consist of up to 93 % of Characeae fragments.
In shallow waters, dense Characeae meadows restrict sediment re-suspension, which consequently reduces the recycling of nutrients to planktonic algae. Moreover, Characeae decompose slower than their vascular counterparts, and the prolonged nutrient storage in their biomass may provide an efficient “nutrient trap” in shallow lakes (Kufel and Kufel 2002).
Materials and methods
Lake Ganau is located 10 km southeast of the town of Ranyia, Sulaimani Governorate, NE Iraq (co-ordinates: 35°51′12″N; 44°45′49″E). It is approximately 350 m in diameter and approximately 20 m deep at its center (Fig. 1). The lake is situated approximately 15 km to the south of the main suture zone of the western Zagros Fold-Thrust Belt that passes through northeastern Iraq close to the border with Iran (Buday 1980; Jassim and Goff 2006). The area mainly consists of high-amplitude anticlines and synclines with the same strike direction (NW–SE) as the Zagros Mountain Belt. Many of the anticlines are asymmetrical, with the southwestern limbs steeper than the northeastern ones.
Physico-chemical parameters of Lake Ganau (Manmi 2008)
SO4 – (ppm)
HCO3 – (ppm)
Maximum T (°C)
Minimum T (°C)
Results and discussion
Based on roundness, sorting, degree of lithification, and preserved microstructures of the grains, three facies were distinguished, as described in the following.
Newly formed lime sand facies
Weathered lime sand facies
Lithified lime sand facies (grainstone)
The duration of the lithification process is not known for the Lake Ganau sediments, but a recently cited example of sub-aerially exposed Holocene oolitic sand on Eleuthera Island, Bahamas, was completely case-hardened by freshwater vadose calcite cementation (Dravis 1996). Although the environmental conditions and characteristics of the sediments from the Bahamas differ from those of Lake Ganau, it can be anticipated that lithification of the oolitic Characeae sediments may be completed within tens of years.
Lake Ganau, a small carbonate-rich freshwater lake in NE Iraq, acts as an effective lime sand factory driven by the vital activity of calcifying characean green algae (genus Chara). Wave action resulting from northeasterly winds assists in the breakdown and sorting of algal skeletons into small fragments (grains) that cover the lake bottom at shallow water depths along the southwestern shore of the lake. The resulting lime sands are characterized by the diagenetic replacement of characean fragments. Moreover, all the grains (bioclasts) are converted to ooids by deposition of concentric micritic laminae, thus leading to thickening of grains and the formation of superficial ooids and micro-oncoids. The lime sands can be classified into three facies that are characterized by increasing sorting, rounding, lithification, and relative age, i.e., diagenetic overprint. The produced sediments are finally transformed into oolitic limestone by lithification forming a grainstone microfacies. This study is the first to record the transformation of all parts of characean algae into loose sediments and, eventually, into laminated carbonate rocks. Lake Ganau thus provides an excellent insight into the occurrence and fossilization process of characean algae and the formation of Characeae-derived carbonates. It may also be considered as a modern reference for the interpretation of ancient freshwater environments.
We are grateful to E. Gierlowski-Kordesch, E. Samankassou, and an anonymous reviewer whose thoughtful comments greatly helped to improve the original manuscript. This research was funded by the German Academic Exchange Service (DAAD; grant no. 50725110).
- Apolinarska K, Pelechaty M, Pukacz A (2011) CaCO3 sedimentation by modern charophytes (Characeae): can calcified remains and carbonate δ13C and δ18O record the ecological state of lakes? A review. Stud Limnol Tel 5:55–66Google Scholar
- Bellen R, Dunnington H, Wetzel R, Morton D (1959) Lexique stratigraphique international III, Fascicule 10a, Asie (Iraq). CNRS Editions, ParisGoogle Scholar
- Brasier M (1980) Microfossils. George Allen and Unwire, LondonGoogle Scholar
- Buday T (1980) Regional geology of Iraq, vol 1. D G Geol Surv Min Invest Publ, BaghdadGoogle Scholar
- Détriché S, Bréhéret JG, Zarki J, Arrat LH, Macaire JJ, Funtugne M (2008) Lake Holocene paleohydrology of Lake Afourgagh (Middle Atlas, Morocco) from deposit geometry and facies. Bull Soc géol France 179:41–50Google Scholar
- Flügel E (2004) Microfacies of carbonate rocks: analysis, interpretation, and application. Springer, Berlin Heidelberg New YorkGoogle Scholar
- Garcia A, Chivas AR (2006) Diversity and ecology of extant and Quaternary Australian charophytes (Charales). Cryptogam Algol 27:323–340Google Scholar
- Ghazala B, Naila B, Shameel M, Shahzad S, Leghar SM (2004) Phycochemistry and bioactivity of two stonewort algae (Charophyta) of Sindh. Pak J Bot 36:733–743Google Scholar
- Jassim SZ, Goff JC (2006) Geology of Iraq. Dolin, Prague, and Moravian Museum, BrnoGoogle Scholar
- Kalkowsky E (1908) Oolith und Stromatolith im norddeutschen Buntsandstein. Z dt geol Ges 60:68–125Google Scholar
- Karim KH, Khanaqa PA, Ameen BM (2010) Types of recent microbialite in slightly acidic spring in Ranyia area, Kurdistan, NE-Iraq. Iraqi Bull Geol Mining 7:27–40Google Scholar
- Kumar A, Oliver R (1984) The occurrence and geological significance of charophyte gyrogonites from the Slippery Rock Formation (Maastrichtian), Central Inlier, Jamaica. Caribb J Sci 20:29–34Google Scholar
- Manmi DA (2008) Water resource management of Ranyia area. NE-Iraq, Dissertation, University of Baghdad, Baghdad, Sulaimanyia areaGoogle Scholar
- Martin-Closas C (2003) The fossil record and evolution of freshwater plants: a review. Geol Acta 1:315–338Google Scholar
- Martin-Closas C, Dieguez C (1998) Charophytes from the Lower Cretaceous of the Iberian Ranges (Spain). Palaeontology 41:1133–1152Google Scholar
- Paul J, Peryt TM, Burne RV (2008) Kalkowsky’s stromatolites and oolites (Lower Buntsandstein, Northern Germany). In: Reitner J, Queric NV, Reich M (eds) Advances in stromatolite geobiology. Lect Notes Earth Sci vol 131, pp 13–28Google Scholar
- Scholle PA, Kinsman DJJ (1974) Aragonitic and high-Mg calcite caliche from the Persian Gulf-a modern analog for the Permian of Texas and New Mexico. J Sediment Res 44:904–916Google Scholar
- Sissakian VK (2000) Geological map of Iraq. Sheets No. 1, Scale 1:1000000, Geological Survey and Mining. GESURV, Baghdad, IraqGoogle Scholar
- Tucker ME, Wright VP (1990) Sedimentology. Blackwell Scientific Publications, OxfordGoogle Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.