Holocene chronostratigraphic beachrocks and their geologic climatic significance

Abstract Rocks that form by cementation in the intertidal parts of beaches are known as beachrock. Nearly all modern beachrocks are restricted to warm climatic belts between latitudes 35°N and 35°S. This suggests that in addition to seawater supersaturated for CaCO 3 , high temperatures within the beach sediments are necessary for cementation. The particles composing beachrocks generally are well-sorted beach sands cemented by carbonate. In this study gently seaward-dipping prograding successions of beachrock have been compared for three different geographic settings: (1) an island in the southern Caribbean offshore from the mainland of Venezuela, part of the Caribbean Plate, (2) the north shore of Puerto Rico, and (3) the shoreline of the rift valley of the Gulf of Aqaba, a steep-sided tectonic valley that forms the northern segment of the Red Sea. The purpose of this study is to determine variables involved in cementing beach sediment into beachrock. Beachrocks are markers of relative sea level. The process of cementation reflects the geochemistry and temperature conditions of the interstitial waters at the time of cementation. Since the seaward-dipping successions reflect progressive stages of cementation, a geologic history of the shoreline may be unraveled. The four- to five levels of prograding beachrock in the three studied areas represent fluctuations in relative sea-level due to eustacy. Beachrocks of the Gulf of Aqaba cements are of decreasing radioactive ages (~7000 yr BP to modern) paralleling increasing strontium isotope ratios (0.708930–0.709248) reflecting chronostratigraphic variation. The strontium isotopic values of these cements are among the highest for Phanerozoic carbonates and point to rapid continental chemical weathering and erosion recording fast unroofing. Superimposed on this orogeny the high values for carbon and strontium isotope ratios may relate to Red Sea sea-floor spreading, related tectonic uplifting, weathering, and erosion. The oxygen isotopic composition of Gulf of Aqaba beachrock, recorded by cement reflects temperature decrease of ambient seawater for approximately half of Holocene time. The computed temperature excursion of the Red Sea beachrock cement implies temperature decrease between the ages 7070 ± 380 and 2620 ± 230 yr BP, an interval of approximately 4 1/2 thousand years, during which the average Red Sea seawater temperature fell from 33 to 17°C. This discovery is at variance with the climate change debate which involves increasing temperatures. Changes in δ 13 C values and 87 Sr/P 86 Sr ratios of the beachrock cement reflect tectonic activity recording uplift and attendant weathering. The levels of prograding beachrock in the three studied areas represent sea-level fluctuations and/or tectonic activity. Earthquakes are indicated for the North American Plate, where at least three levels of beachrock were generated during the last few hundred years. The 87 Sr/P 86 Sr ratios of the beachrock cements of the southern Caribbean offshore islands of Venezuela (Los Roques Island) are above that of modern seawater. By contrast the 87 Sr/P 86 Sr ratios of Puerto Rico samples overlap those of modern seawater, and the Red Sea values are below that of modern seawater. The increase in the 87 Sr/P 86 Sr ratios of the beachrock cement with decreasing age suggests that weathering ratios were high or that the weathered rocks were exceptionally radiogenetic, or reflect changes in the global isotope mass balance.