ALBERT

Description: The dataset content the 14C and 210Pb geochronologies of mangroves, seagrass and saltmarshes of the Central eastern red sea coast and western Arabian Gulf. It also content depth profiles of CaCO3 content in the sediments of these ecosystems as well as the calculated burial rates in those sediments. A total of 52 cores were sampled along 80 km coastline of the Kingdom of Saudi Arabia in the central Red Sea (2015) and the Arabian Gulf (2016) (29 in mangroves forests and 23 in seagrass meadows, table 1). Seagrass soil cores were sampled in 1 to 4 m deep seagrass meadows, dominated by Halophila sp., Thalassia hemprichii, Enhalus acoroides, Thalassodendrum ciliatum and Cymodeacea sp., and mangrove soil cores were sampled in intertidal mono-specific A. marina forests. The sites sampled in the Red Sea include the coastal island of Al Taweelah (22°16'N, 39°05'E; 8 mangrove and 3 seagrass coring locations) and 3 coastal lagoons: Khor Almesena'a (22°22'N, 39°07'E; 8 mangroves and 10 seagrass coring locations), Khor Al-Baqila (22°44'N, 39°00'E; 6 coring locations in mangroves) and Khor Al-Kharrar (22°57'N, 38°51'E; 7 coring locations in mangroves and 10 in seagrasses) (Fig 1). Khor Al-Baqila underwent a major alteration with the conversion of the entire southern side of the embayment into a petrochemical terminal starting in 1981. Similarly, an important development of hard engineering structures occurred in the shoreline in front of Al Taweelah Island in the early 2010's . We also sampled 25 cores in the Saudi Arabian coast of the Arabian Gulf at 4 sites, Ras Safaniya (27°58'N, 48°46'E), Abu Ali Island (27°17' N, 49°33' E), Ras Tanura - Safwa (26°41' N, 50°00' E) and Uqair (25°43' N, 50°13' E) (Fig. 1). Three seagrass cores were sampled at each location in Halodule uninervis and Halophila stipulacea meadows (total of 12 cores). Additionally, 3 mangrove and 3 sabkha cores were sampled in the south of Abu Ali Island and 4 mangrove cores and 3 sabkha cores were sampled in the area of Ras Tanura - Safwa (Fig 1). All sites except Uqair have underwent alterations associated with the prevalent industrial and urban development in the region since the 1950s, including land reclamation, dredging, construction of bridges and pipelines, and oil spills. Soil cores were sampled using manual percussion and rotation (PVC pipe with an inner diameter of 70 mm). The length of core barrel inserted into the soil and the length of retrieved soil was recorded in order to correct for compression effects following the guidelines of Howard et al. (2014). All variables studied here are referenced to the corrected, uncompressed depths. The cores were sealed at both ends, transported vertically and stored at 4 °C before processing in the laboratory. 2.3 Biogeochemical analysis The soil cores were segmented into 1-cm-thick slices, which were oven-dried at 60 °C until constant weight to determine the dry bulk density (g cm-3). The slices were then grounded in an agate mortar and subdivided for analysis. All depths were corrected for compression considering a uniform distribution of the compaction throughout the total length of the cores as described by Serrano et al. (2016b). The mean ± SE compression factors (depth ratio between compressed and uncompressed soil) were 1.05 ± 0.18, 1.04 ± 0.03 and 1.16 ± 0.11 in the cores from sabkha, mangrove and seagrass sites of the Arabian Gulf respectively, and 1.19 ± 0.14 and 1.23 ± 0.17 in the seagrass and mangrove cores of the Red Sea. Short-term (last decades - century) and long-term (millennia) soil chronologies were established using 210Pb and 14C analyses, respectively. Forty-five cores were analyzed to retrieve chronologies using the 210Pb technique, 20 cores from the Red Sea (9 in seagrass and 11 in mangroves) and 25 cores from the Arabian Gulf (7 in mangroves, 12 in seagrass and 6 in sabkhas). The activity concentrations of 210Pb in the upper 20 to 30 cm (compressed) were determined in the soil fraction 〈0.125 mm by alpha spectrometry through the measurement of its granddaughter 210Po, assuming radioactive equilibrium between both radionuclides (Sanchez-Cabeza et al., 1998). The activity concentrations of excess 210Pb used to obtain the age models were determined as the difference between total 210Pb and 226Ra (supported 210Pb). Concentrations of 226Ra were determined for selected samples along each core by low-background liquid scintillation counting method (Wallac 1220 Quantulus) adapted from Masqué et al. (2002). These activity concentrations were found to be comparable with the concentrations of total 210Pb at depth below the excess 210Pb horizons. Analyses of reagent blanks, replicates and a reference material (IAEA-315, marine soils) were carried out for both 210Pb and 226Ra to assess for any contamination and to ensure reproducibility of the results. Average soil mass accumulation rates (MAR, expressed in g DW cm?2 yr?1) for the last decades/century were estimated using the Constant Flux: Constant Sedimentation model (CF:CS, Krishnaswamy et al., 1971). MAR were transformed into SAR (cm yr-1) using the dry bulk density (DBD) of each core. A total of 179 radiocarbon analyses were conducted in 77 cores (25 cores from the Arabian Gulf and 52 from the Red Sea) by Accelerator Mass Spectrometry. Analyses were done at 2 soil depths per core in the Red Sea cores and 3 depths in the Arabian Gulf cores, following standard procedures (ISO 17025 and ISO 9001) at the AMS Direct Laboratory, USA. Samples consisted of either pooled shells or bulk soils. The raw radiocarbon dates reported by the laboratory were calibrated using the R routine "Bacon" for Bayesian chronology building (Blaauw & Christen, 2011), assuming marine reservoir corrections of 110 ± 38 and 180 ± 53 years for the Red Sea and the Arabian Gulf, respectively (Southon et al., 2002). From the Bacon routine output, the mean age was used to produce an age-depth weighted regression model forced through 0 (0 cm is 1950 BP), using as weight the sum of the Euclidean distances of the minimum and maximum ages. We report the slope ± SE of the regression as the corresponding long-term SAR. 2.4 Carbonate content and Cinorg burial rates Calcium carbonate (CaCO3) content was determined in every fourth to fifth cm from surface to 20 cm depth. CaCO3 measurements were done with a calcimeter (Pressure Gauge Model 432, Fann, Houston, TX, USA; ASTM D 4373-84 Standard) by reacting the CaCO3 present in the sample with 10% HCl in a sealed reaction cell. The pressure build up due to the CO2 was measured with a bourdon tube pressure gauge that was pre-calibrated with reagent grade CaCO3.