Claire Seard

Reef response to sea-level and environmental changes during the last deglaciation: Integrated Ocean Drilling Program Expedition 310, Tahiti Sea Level

The last deglaciation is characterized by a rapid sea-level rise and coeval abrupt environmental changes. The Barbados coral reef record suggests that this period has been punctuated by two brief intervals of accelerated melting (meltwater pulses, MWP), occurring at 14.08–13.61 ka and 11.4–11.1 ka (calendar years before present), that are superimposed on a smooth and continuous rise of sea level. Although their timing, magnitude, and even existence have been debated, those catastrophic sea-level rises are thought to have induced distinct reef drowning events. The reef response to sea-level and environmental changes during the last deglacial sea-level rise at Tahiti is reconstructed based on a chronological, sedimentological, and paleobiological study of cores drilled through the relict reef features on the modern forereef slopes during the Integrated Ocean Drilling Program Expedition 310, complemented by results on previous cores drilled through the Papeete reef. Reefs accreted continuously between 16 and 10 ka, mostly through aggradational processes, at growth rates averaging 10 mm yr−1. No cessation of reef growth, even temporary, has been evidenced during this period at Tahiti. Changes in the composition of coralgal assemblages coincide with abrupt variations in reef growth rates and characterize the response of the upward-growing reef pile to nonmonotonous sea-level rise and coeval environmental changes. The sea-level jump during MWP 1A, 16 ± 2 m of magnitude in ∼350 yr, induced the retrogradation of shallow-water coral assemblages, gradual deepening, and incipient reef drowning. The Tahiti reef record does not support the occurrence of an abrupt reef drowning event coinciding with a sea-level pulse of ∼15 m, and implies an apparent rise of 40 mm yr−1 during the time interval corresponding to MWP 1B at Barbados.

Genesis of microbialites as contemporaneous framework components of deglacial coral reefs, Tahiti (IODP 310): COMMENT

The microbialite contribution to the volume and rigidity of carbonate buildups and reefs have often equaled, or exceeded, the contribution of skeletal metazoans throughout the geological column (e.g., see Webb 1996). Since their Wrst descriptions in the Late Pleistocene to Holocene reefs from Tahiti (Montaggioni and Camoin 1993; Camoin and Montaggioni 1994), microbialites have been reported in various Quaternary reef frameworks from a number of areas, as well as from the walls of the deeper forereef slopes and in shallow-water caves (see review in Camoin et al. 2006). This implies that microbialites may have played a signiWcant role in the development of Quaternary reefs, as well as in related sedimentary and diagenetic processes. The accurate reconstruction of development patterns and the quantiWcation of microbialite growth rates in carbonate buildups and reefs clearly rely on the ability to obtain a reliable chronological frame. The sedimentological and paleoecological criteria (e.g., the relationships between builders, environmental signiWcance of the various reefdwelling communities, etc.) provide a relative chronology which, generally, does not allow to fully address those issues; major controversies regarding reef growth patterns and relationships between actual builders and microbial fabrics have appeared in the past and are still under debate. Quaternary reefs provide the unique opportunity to address those issues through the potential dating of the various components of the reef frameworks (i.e., corals, coralline algae, and microbialites in the case of Tahiti) coupled with a proper study of the framework architecture. A Wrst attempt to obtain reliable ages on microbialites using U-series has been made a few years ago (Camoin et al. 2006). The major objectives of the Westphal et al. (2010) paper were to examine the relative timing of coral framework development and microbialite encrustation in Tahiti IODP cores (Camoin et al. 2007a, b), and to reconsider the previous interpretations based on the study of onshore drill cores and dredged samples (Montaggioni and Camoin 1993; Camoin and Montaggioni 1994; Camoin et al. 1999, 2006). This comment aims to discuss brieXy the results (ages and observations) and the interpretations presented in the Westphal et al. (2010) paper, especially in light of recent results obtained on the same drill cores by using an array of various methods (Seard et al. 2011). We do not discuss below the “Origin of microbialites” section of that paper in which Westphal et al. agree with our previous interpretations (see e.g., Camoin et al. 1999). Ages—In our view, the Westphal et al. (2010) paper does not include an appropriate chronological frame to properly address the timing of coral framework development and microbialite encrustation, as well as the evolution of environmental conditions during reef growth for the following reasons:

Continental Carbonates Reservoirs: The Importance of Analogues to Understand Presalt Discoveries

Recent discoveries offshore Brazil have induced a renewal of interest in the study of recent and ancient continental carbonate systems which developed in a wide range of depositional settings, reflecting aerial to subaqueous environments. Recent and ancient continental carbonate analogs provide some keys to depict the sedimentologic/sequential pattern observed at the core scale and help in the understanding of the impact of climate change, fluid flow and water chemistry on the carbonate factory. It is noteworthy that the widespread microbial development in continental carbonate systems occurs in stratigraphic intervals typified by specific climatic and geodynamic conditions, and sometimes coincides with similar development in the marine realm. Stromatolites are more developped in high water level condition. But comparative studies between intracratonic (Recent Great Salt Lake; Eocene Green River lacustrine systems) and rift lacustrine systems demonstrates that they are more extensive on a flat substrate. The control exerted by the topography may increase during abrupt alternations of arid and humid periods, influencing the water chemistry and, accordingly, leading to the development of anoxic and/or evaporitic conditions. The key issue is therefore to understand the development of carbonate in lacustrine condition, how the sedimentary bodies and features can be preserved, and how their good reservoir properties can be maintained. High subsidence rate will influence the preservation potential of the relevant carbonate bodies, while the geothermal gradient, water chemistry or volcanic activity will impact the reservoir properties. In addition, meteoric or thermogenic travertine deposits, are an additional carbonate product that must be considered in the evaluation of continental carbonate reservoir systems.