B. L AZAR , M. YEHUDAI , N. BAR , Y. SHAKED , A ...slabs, parallel to growth direction (red arrows)....
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B. LAZAR 1 , M. YEHUDAI 1,2 , N. BAR 1 , Y. SHAKED 3 , A. AGNON 1 AND M. STEIN 2 1 Institute of Earth Science, The Hebrew University of Jerusalem, ISRAEL 91904 ([email protected]) 2 Geological Survey of Israel, 30 Malkhe Israel St. Jerusalem, ISRAEL,95501 ; 3 The Interuniversity Institute for Marine Sciences, Eilat, ISRAEL 88103 Abstract Pristine aragonite skeletons of reef building corals can be rapidly recrystallized to calcite by the interaction of the corals with freshwater in coastal aquifers. The aragonite/ calcite transformation is accompanied by opening the coral’s U-Th isotope system in which uranium is partly lost while Th remains adsorbed and reincorporates into the newly formed calcite. Depending on the geological setting of the reef, the corals may incorporate secondary aragonite with higher U and 234 U/ 238 U isotope ratio, while still submerged, before the recrystallization process. The recrystallization occurs during sea level drop or coast tectonic uplift and later may follow a subaerial closed system decay scheme. In this study we examine the behavior of the U and Th in calcitic corals from the last interglacial reefs at the northern Gulf of Aqaba (Fig. 1). We analyzed several subsamples from selected reef coral skeletons in an attempt to follow the recrystallization scheme of the corals and find a reliable method to estimate the age of these heavily altered corals (Figs. 2, 3). The main assumptions were that all subsamples from the same coral have identical deposition age and the subsample’s Th (and hence 230 Th) was fully preserved during recrystallization to calcite (increasing the 230 Th/ 238 U isotope ratio). The estimate of the age of corals’ recrystallization bares an important paleoenvironmental information because it dates time when the hydrological conditions of the currently hyperarid northern Red Sea were wetter than present. Fig. 2: A composite cross-section of the fossil reef terraces R0-R3, compiled from all exposures seen at the study-area (Fig. 1). R0 is exposed at sea-level along the shore-line. The corals collected from there are 100% aragonite. R1 is only exposed in area B (Fig. 1). As the coral sample taken from that terrace is composed of~90% aragonite, we assume that 100 ka is the true deposition age of terrace R1. “Scholz LIG (Last Interglacial)” relates to aragonitic corals from terraces that were described and dated by Scholz et al. (2004). R2 and R3 coral samples are 100% calcite and therefore, the ages calculated for these terraces are considered diagenesis ages (see Fig. 4). R0~ 6 ka (100% aragonite) R2~ 100 ka (rec. age) (100% calcite) E W Modern reef Road R0 R2 R3 R1 ~100 ka (~90% Aragonite) Scholz LIG (aragonite corals) R3 ~ 130 ka (rec. age) (100% calcite) Fig. 1: Maps of the study areas located in the northern Gulf of Aqaba. Different levels of Sub-buried fringing reef tables are marked in different colors and numbers. The reef terraces were mapped by Bar et al. (2013). N S N S B B Fig. 5: A model presenting the interaction of a reef-framework with marine and terrestrial groundwater, during sea-level drop (Lazar and Stein, 2011). W E Fresh Groundwater Fringing Reef Vadose Zone Red Sea Saline Groundwater 125 ka W E Fresh Groundwater Red Sea Vadose Zone Saline Groundwater Emerged Reef 100 ka New Fringing Reef Area of marine/subaerial diagenesis Sealevel drop Isotopic measurements were performed using a multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS) Nu Instruments Ltd. (UK) at the Geological Survey of Israel. Methods 16 samples of fossil corals were collected from four reef terraces exposed at the study area (Figs. 1,3). The collected samples were processed for U-Th isotope analysis (Fig. 4). Fig. 3: Coral sample after being cut and sliced to thin slabs, parallel to growth direction (red arrows). Results and discussion Data from calcitic coral samples (R2) scatter on the evolution diagram (Fig. 4A), while aragonite corals (Scholz_LIG) show a linear correlation along the 120 ka isochron. The data of the calcitic samples is plotted on a [ 230 Th] vs. [ 238 U] diagram (Fig. 4B). The slope and intercept of the correlation lines are used to calculate the ages of diagenesis and coral formation (Fig. 4B-inset). Conclusions U-Th ages of both the aragonite coral formation and the recrystallization of aragonite to calcite can be derived from the slope and intercept of the correlation- lines in the 230 Th and 238 U property-property (e.g. activity) diagrams (Fig. 4) Wet conditions prevailed in the Gulf of Aqaba at ~ 100 ka BP after sea level dropped from its MIS5e high stand. The reef terraces were saturated with freshwater from the coastal aquifers (Fig. 5) Fig. 4: A- Evolution diagram of the U-Th system, used for closed system age calculation. The calcitic corals data is scattered, while the aragonite corals show linear correlation. B- Property vs. property diagram of 230 Th and 238 U activity. The slopes of the lines are used in diagram A to calculate the age of recrystallization. The inset in B shows a U-Th evolution model of a hypothetical clacitic coral from its initial formation as an aragonite coral (green diamond), through recrystallization to calcite (from green to brown diamonds), to present time (blue diamonds). The scattering of the brown diamonds reflects the U loss during recrystallization. Recrystallization age is calculated from the slope and intercept of the property–property plots: 230 Th vs 238 U activities and 234 U vs 238 U activities (verbal explanation will be provided by the authors at the poster session).
Transcript of B. L AZAR , M. YEHUDAI , N. BAR , Y. SHAKED , A ...slabs, parallel to growth direction (red arrows)....
B. LAZAR1, M. YEHUDAI1,2, N. BAR1, Y. SHAKED3, A. AGNON1 AND M. STEIN2 1Institute of Earth Science, The Hebrew University of Jerusalem, ISRAEL 91904 ([email protected])
2Geological Survey of Israel, 30 Malkhe Israel St. Jerusalem, ISRAEL,95501 ; 3The Interuniversity Institute for Marine Sciences, Eilat, ISRAEL 88103
Abstract Pristine aragonite skeletons of reef building corals can be rapidly recrystallized to calcite by the interaction of the corals with freshwater in coastal aquifers. The aragonite/calcite transformation is accompanied by opening the coral’s U-Th isotope system in which uranium is partly lost while Th remains adsorbed and reincorporates into the newly formed calcite. Depending on the geological setting of the reef, the corals may incorporate secondary aragonite with higher U and 234U/238U isotope ratio, while still submerged, before the recrystallization process. The recrystallization occurs during sea level drop or coast tectonic uplift and later may follow a subaerial closed system decay scheme. In this study we examine the behavior of the U and Th in calcitic corals from the last interglacial reefs at the northern Gulf of Aqaba (Fig.1). We analyzed several subsamples from selected reef coral skeletons in an attempt to follow the recrystallization scheme of the corals and find a reliable method to estimate the age of these heavily altered corals (Figs. 2, 3). The main assumptions were that all subsamples from the same coral have identical deposition age and the subsample’s Th (and hence 230Th) was fully preserved during recrystallization to calcite (increasing the 230Th/238U isotope ratio). The estimate of the age of corals’ recrystallization bares an important paleoenvironmental information because it dates time when the hydrological conditions of the currently hyperarid northern Red Sea were wetter than present.
Fig. 2: A composite cross-section of the fossil reef terraces R0-R3, compiled from all exposures seen at the study-area (Fig. 1). R0 is exposed at sea-level along the shore-line. The corals collected from there are 100% aragonite. R1 is only exposed in area B (Fig. 1). As the coral sample taken from that terrace is composed of~90% aragonite, we assume that 100 ka is the true deposition age of terrace R1. “Scholz LIG (Last Interglacial)” relates to aragonitic corals from terraces that were described and dated by Scholz et al. (2004). R2 and R3 coral samples are 100% calcite and therefore, the ages calculated for these terraces are considered diagenesis ages (see Fig. 4).
R0~ 6 ka (100% aragonite)
R2~ 100 ka (rec. age) (100% calcite)
E W
Modern reef
Road R0
R2
R3
R1 ~100 ka (~90% Aragonite)
Scholz LIG (aragonite corals)
R3 ~ 130 ka (rec. age) (100% calcite)
Fig. 1: Maps of the study areas located in the northern Gulf of Aqaba. Different levels of Sub-buried fringing reef tables are marked in different colors and numbers. The reef terraces were mapped by Bar et al. (2013).
N
S
N S
B
B
Fig. 5: A model presenting the interaction of a reef-framework with marine and terrestrial groundwater, during sea-level drop (Lazar and Stein, 2011).
W E
FreshGroundwater
Fringing Reef Vadose Zone
RedSea
SalineGroundwater
125 ka
W E
FreshGroundwater
Red Sea
Vadose Zone
SalineGroundwater
EmergedReef
100 ka
New FringingReef
Area of marine/sub-‐aerial diagenesis
Sea-‐level drop
Ø Isotopic measurements were performed using a multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS) Nu Instruments Ltd. (UK) at the Geological Survey of Israel.
Methods Ø 16 samples of fossil corals were collected
from four reef terraces exposed at the study area (Figs. 1,3). The collected samples were processed for U-Th isotope analysis (Fig. 4).
Fig. 3: Coral sample after being cut and sliced to thin slabs, parallel to growth direction (red arrows).
Results and discussion Ø Data from calcitic coral samples (R2) scatter on the evolution diagram (Fig. 4A), while aragonite corals
(Scholz_LIG) show a linear correlation along the 120 ka isochron. Ø The data of the calcitic samples is plotted on a [230Th] vs. [238U] diagram (Fig. 4B). The slope and intercept of the
correlation lines are used to calculate the ages of diagenesis and coral formation (Fig. 4B-inset).
Conclusions Ø U-Th ages of both the aragonite coral formation and the recrystallization of
aragonite to calcite can be derived from the slope and intercept of the correlation-lines in the 230Th and 238U property-property (e.g. activity) diagrams (Fig. 4)
Ø Wet conditions prevailed in the Gulf of Aqaba at ~ 100 ka BP after sea level dropped from its MIS5e high stand. The reef terraces were saturated with freshwater from the coastal aquifers (Fig. 5)
Fig. 4: A- Evolution diagram of the U-Th system, used for closed system age calculation. The calcitic corals data is scattered, while the aragonite corals show linear correlation. B- Property vs. property diagram of 230Th and 238U activity. The slopes of the lines are used in diagram A to calculate the age of recrystallization. The inset in B shows a U-Th evolution model of a hypothetical clacitic coral from its initial formation as an aragonite coral (green diamond), through recrystallization to calcite (from green to brown diamonds), to present time (blue diamonds). The scattering of the brown diamonds reflects the U loss during recrystallization. Recrystallization age is calculated from the slope and intercept of the property–property plots: 230Th vs 238U activities and 234U vs 238U activities (verbal explanation will be provided by the authors at the poster session).
