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The development of benthic foraminiferal assemblages during the past 6,000 yrs was investigated in Holocene sediment cores from three carbonate platforms (Turneffe Islands, Lighthouse Reef, and Glovers Reef) of Belize, Central America. Foraminiferal assemblages and their diversity were determined in different time periods to identify their dependence on environmental factors, such as lagoonal age, lagoonal depth, water circulation, substrate, bottom-water temperature, and salinity. Geochemical proxies (δ18O and δ13C), obtained from the common larger foraminifer Archaias angulatus were used to estimate Holocene seasonal BW-temperatures and climate variabilities. A total of 51 samples were taken from 12 vibracores for taxonomic determination and 10 to 15 subsamples of 32 tests of Archaias angulatus were used for stable oxygen and carbon isotope analyses. Based on cluster analyses, seven benthic foraminiferal assemblages are distinguished during the Holocene. The three platforms exhibit characteristic differences in benthic foraminiferal fauna and diversity, which are controlled by their respective environments during the last 6,000 yrs. Turneffe Islands has four benthic foraminiferal assemblages, which are typical for restricted lagoons with fluctuating salinity. Lighthouse Reef is inhabited by two benthic foraminifera associations, which are characteristic of high water exchange with the surrounding ocean and clear waters. Glovers Reef is characterized by two benthic foraminiferal assemblages, which occur in deeper lagoons with slow water circulation. In general, during the Holocene, the highest mean diversity, evenness, and richness of benthic foraminifera were found in the Turneffe Islands and the lowest occurred at Glovers Reef. The foraminiferal faunas of the Lighthouse and Glovers Reefs had been in a “Diversification Stage” since 6,000 yrs, whereas the foraminiferal fauna of the Turneffe Islands reflects the development from a “Colonisation” (~4,000 yrs BP) to a “Diversification Stage” (~2,000 yrs to present time). Lagoonal depth, water circulation, substrate, and BW-temperature have higher influence on foraminiferal diversity as compared to lagoonal size and age. The negative correlation between diversity and lagoonal depth is based on differences in light intensity and substrate. In contrast to Lighthouse Reef, the Turneffe Islands and Glovers Reef show decreasing diversity of benthic foraminifera with increasing lagoon depth, due to finer sediment, turbid waters and/or dense mangrove growth, which reduce the light intensity and the number of species. Water Circulation also affected the benthic foraminifera modes of living and their diversity during the last 6,000 yrs. Increasing abundances of infaunal taxa refer to restricted circulation and/or lower oxygen conditions, as assumed for the Turneffe Islands and Glovers Reef. Increasing abundances of epifaunal foraminifera, as observed in the Lighthouse Reef indicate better circulation and/or higher oxygen conditions. Holocene BW-temperature reconstructions based on δ18O of single Archaias angulatus tests do not correspond to typical Holocene climate models of the Caribbean. In the Belize area, mean BW-temperature trends indicate local climate variations. A decrease of δ13C values during the last 1,000 yrs could be related to the “Suess Effect”. The seasonal BW-temperature variations within single large benthic foraminifera tests correspond to present-day temperature fluctuations in the lagoons, and indicate higher temperatures in Summer and Autumn and lower temperatures in Winter and Spring.
Die drei Atolle Glovers Reef, Lighthouse Reef und Turneffe Islands vor der belizischen Küste im Karibischen Meer unterscheiden sich in Geomorphologie, Lagunentiefe, Sedimentbeschaffenheit, Mangroven- und Seegrasbewuchs, Wellen- und Strömungseinfluss sowie in ihren Sedimentationsraten und ihrem Entstehungsalter. Um herauszufinden, ob die Bivalven-Vergesellschaftungen verschiedener Lagunenzonen diese Unterschiede widerspiegeln, wurden 32 bis 44 rezente Sedimentproben auf jedem Atoll entnommen (Gesamtprobenzahl: 111). Deren Datensatz von insgesamt 32 122 Bivalvenschalen wurde anschließend Q-Mode-Cluster-Analysen unterzogen. Neben der Verteilung charakteristischer Arten wurde auch die Verteilung von Bivalven unterschiedlicher Lebens- und Ernährungsweise untersucht. Chione cancellata, ein flach grabender Suspensionsfresser, besiedelt bevorzugt (1) flache, wellen- und strömungsbeeinflusste Lagunenzonen. Die Sedimente (2) sehr hoch energetischer Flachwasserbereiche enthalten zudem hohe Anteile tiefer grabender Suspensionsfresser der Gattung Ervilia. Im (3) Rückriffbereich und am Atollrand sind tief grabende, Detritus fressende Telliniden häufig. Gouldia cerina, wie Chione ein flach grabender Suspensionsfresser, ist typisch für (4) geschlossene Flachwasserbereiche, während die Chemosymbionten-tragende, ebenfalls flach grabende Parvilucina sp. A. vorwiegend in (5) geschlossenen, tiefen Lagunenzonen vorkommt. Charakteristisch für (6) Mangrovengebiete ist Crassinella lunulata, ein sehr flach grabender Suspensionsfresser. Die Anteile taphonomischer Signaturen auf den Schalen, wie Bohrspuren, Inkrustationen, Fragmentierung und Abrasion sowie Diversität, Evenness und Richness sind auf Glovers Reef am höchsten und nehmen über Lighthouse Reef nach Turneffe Islands ab. Da in die gleiche Richtung zunehmende Sedimentationsraten auf den drei Atollen zu verzeichnen sind (GISCHLER 2003), ist vermutlich der abnehmende Effekt des Time-averaging für diesen Trend verantwortlich. Neben der rezenten Fauna wurden auch die Bivalven aus Vibrationsbohrkernen (ein Kern von jedem Atoll) untersucht. Die fossilen Bivalven-Vergesellschaftungen der inneren Lagunen von Glovers Reef, Lighthouse Reef und Turneffe Islands zeigen seit deren Entstehung eine für das jeweilige Atoll typische Fauna, die sich seit ~7000 YBP weiter entwickelte. Sie reflektieren damit die bereits im Anfangsstadium charakteristischen Unterschiede der drei Atolle.