The EUCFe cruise (RV Kilo Moana, 2006) was designed to characterize sources of Fe to the western equatorial Pacific and its transport by the Equatorial Undercurrent (EUC), a narrow and fast eastward current flowing along the equator, to the eastern equatorial Pacific High Nutrient Low Chlorophyll (HNLC) region. This study presents seawater dissolved (DFe) and particulate (PFe) iron concentrations and isotopic compositions (delta 56DFe and delta 56PFe) from 15 stations in the equatorial band (2 degrees N-2 degrees S) between Papua New Guinea and 140 degrees W, over more than 8500 km along the equator and in the upper 1000 m of the water column.delta 56DFe and delta 56PFe ranged from -0.22 parts per thousand to +0.79 +/- 0.07 parts per thousand and from -0.52 parts per thousand to +0.43 +/- 0.07 parts per thousand, respectively (relative to IRMM-14, 95 % confidence interval). Source signatures, biogeochemical processes and transport all contribute to these observations. Two distinct areas, one under continental influence (the western equatorial Pacific) and an open ocean region (the central equatorial Pacific), emerged from the data. In the area under continental influence, high PFe concentrations along with delta 56DFe values systematically heavier than that of delta 56PFe indicated an equilibrium fractionation and the co-occurrence of chemical fluxes from both phases toward the other. This exchange occurs through non-reductive processes, as previously proposed from three of the eight stations of this area (Labatut et al., 2014) and extends up to 1200 km from the coast. In the open ocean area, preservation of a DFe isotopic signature of similar to +0.36 parts per thousand within the EUC, from Papua New Guinea to the central equatorial Pacific (7800 km), confirmed the origin of the DFe carried within this current toward the HNLC region. At the same depth, bordering the EUC at 2 degrees N and 2 degrees S at 140 degrees W, light isotopic signatures suggested that iron was originating from the eastern Pacific oxygen minimum zones. These light signatures were also observed in deeper central waters, between 200 and 500 m. Our data did not allow conclusions about fractionation during uptake by phytoplankton, but indicated that any fractionation, if present, must be small, no larger than a few tenths of a per mil.

Abstract. This study presents aerosol iron isotopic compositions (δ56Fe) in Western and Central Equatorial and Tropical Pacific Ocean. Aerosols supply iron (Fe), a critical element for marine primary production, to the open ocean. Particulate aerosols, > 1 µm, were sampled during EUCFe cruise (RV Kilo Moana, PI: J. W. Murray, 2006). One aerosol sample was isotopically lighter than the crust (δ56Fe=-0.16 ± 0.07 ‰, 95 % confidence interval), possibly originating from combustion processes. The nine other aerosol samples were isotopically heavier than the crust, with a rather homogeneous signature of 0.31 ± 0.21 ‰ (2SD, n=9). Given i) this homogeneity compared to the diversity of their modeled geographic origin and ii) the values of the Fe/Ti ratios used as a lithogenic tracer, we suggest that these heavy δ56Fe signatures reflect isotopic fractionation of crustal aerosols caused by atmospheric processes. Using a fractionation factor of Δsolution - particle=-1.1 ‰, a partial dissolution of ≈20 % of the initial aerosol iron content, followed by the removal of this dissolved fraction, would explain the observed slightly heavy Fe isotope signatures. Such fractionation has been observed previously in laboratory experiments, but never before in a natural environment. The removal of the dissolved fraction of the aerosols has not been previously documented either. This work illustrates the strong constrains provided by the use of iron isotopes for atmospheric process studies.

Abstract This work evaluates glacial dust as a source of sediment, and associated iron (Fe), to the Fe‐limited Gulf of Alaska (GoA). A reanalysis of GoA sediment data, using rare earth elements and thorium as provenance tracers, suggests a flux to the ocean surface of Copper River (AK) glacial dust, and associated Fe, that is comparable to the flux of dust from Asia, at least 1,000 km from the narrow mountain valley glacial dust source area. This work suggests dust from Asia may not be the largest source of Fe to the GoA. Dust models fail to accurately simulate this glacial dust transport because their coarse resolution underestimates wind speeds, and the dust flux. This work suggests that glacial dust fluxes may have been important in the geologic past (e.g., the last glacial maximum) from locations where there was more extensive coverage by glaciers than at present.

The Eastern Pacific Warm Pool (EPWP) modulates global climate through its connection with tropical Pacific circulation, but sparse paleoceanographic data from this region limits our understanding of its role in past climate variability. We present a 144 kyr alkenone‐sea surface temperature (SST) reconstruction from core NH22P, located in the northern EPWP, that shows local warming occurred during periods of global cooling. Climate model simulations of the Last Glacial Maximum indicate that both ice sheet and greenhouse gas forcing slowed wind speeds over the EPWP, which attenuated glacial cooling of local SST via the wind‐evaporation‐SST feedback. Spectral analysis further suggests precessional pacing of the warming spikes. Vernal equinox insolation could explain this pacing as direct shortwave heating during boreal spring would have contributed to the early seasonal intensification of the EPWP. This work provides crucial constraints on tropical Pacific glacial climate variability and highlights the unique response of the EPWP to global climate forcings.

Dissolved inorganic carbon (DIC) and total alkalinity (TA) were sampled in December 2018 and May 2019 in the Exclusive Economic Zone (EEZ) of Qatar in the Arabian Gulf. pCO(2), pH and CO32- were calculated from DIC and TA. TA, DIC and salinity increase in the Gulf due to evaporation after entering through the Strait of Hormuz. Temperature also increases. The pCO(2) in surface seawater averaged 458 +/- 62 which was higher than the atmospheric value of 412 ppm. Hence, the Gulf was a source of CO2 to the atmosphere. pCO(2) in seawater is controlled by TA relative to DIC as well as temperature and salinity. A hypothetical model calculation was used to estimate how much pCO(2) could increase in surface seawater due to various processes after entering through the Strait of Hormuz. Increases in T and S, in the absence of biogeochemical processes, would increase pCO(2) to 537 mu atm, more than enough to explain the high pCO(2) observed. CO2 is lost from the Gulf due to gas exchange, decreasing DIC, and reducing pCO(2) to 464 mu atm, similar to that observed. The impact of biological processes depends on the process: calcification increases pCO(2) while net primary production decreases pCO(2). Salinitynormalized (to S = 40) total alkalinity (NTA) and dissolved inorganic carbon (NDIC) in surface seawater decrease as waters flow north from Hormuz. The slope suggests that removal of C as CaCO3, organic matter (CH2O) or gas exchange (F-CO2) is occurring with a ratio of DCaCO3/(Delta CH2O or F-CO2) = 1:2.86. The tracer Alk*, defined as the deviation of potential alkalinity (A(P)) (where A(P) = TA + 1.26 [NO3]) from conservative potential alkalinity ((A(p)(C)), (A(p)(C) = S A(p)/S ' where A'(P) and S' are mean values for the whole surface ocean) has values primarilydetermined by CaCO3 precipitation and dissolution. Its values in the Gulf ranged from -50 to - 310 mu mol kg(-1) implying CaCO3 precipitation. The average value of DAlk*, the difference in Alk* between specific locations in the Qatari EEZ and the surface water entering through the Strait of Hormuz, was -130 mu mol kg(-1) which corresponded to a calcification of 65 mu mol kg(-1). Our model calculations indicate that this would increase pCO(2) to 577 mu atm. Carbonate forming plankton have not been observed in the water column suggesting that calcification occurs in corals, even though they have been severely damaged by past bleaching events. The amount of DIC removed by net primary production is small, consistent with an oligotrophic food web dominated by remineralization. It appears that the role of biological production in the water column for the control of pCO(2) is very small. The high observed pCO(2) reflects a balance between sources due to the impact of increasing T and S on the carbonate system equilibrium constants and net calcification and sinks due to CO2 loss due to gas exchange and net primary production in surface seawater after it enters the Gulf through the Strait of Hormuz.

Dissolved inorganic carbon (DIC) and total alkalinity (TA) were sampled in December, 2018 and May, 2019 in the Exclusive Economic Zone (EEZ) of Qatar in the Arabian Gulf. pCO2 calculated in surface seawater averaged 459 ± 61 matm and was supersaturated with respect to the atmosphere. The region was degassing CO2 to the atmosphere and the flux was about 1.25 mmol C m-2 d-1. The origin of this excess CO2 must be due to CaCO3 precipitation. The horizontal relationship between salinity-normalized total alkalinity (NTA) and dissolved inorganic carbon (NDIC) showed that CaCO3 formation was more important, relative to net biological productivity, than in the open ocean. The tracer Alk* has values primarily determined by CaCO3 formation and values of Alk* ranged from -50 to -310 mmol kg-1, which is consistent with substantial CaCO3 formation. DAlk* increased with increasing distance northward from Hormuz. The rate of calcification calculated from the air-sea flux of CO2 (5.6 mmol C kg-1 y-1) and from DAlk* (5.9 mmol C kg-1 y-1) agreed well. However, CaCO3 formation by net calcification in coral reefs is unlikely as they have limited distribution and have been severely damaged by past coral bleaching. There are high concentrations of excess particulate Ca in the water column that cannot be accounted for by input of CaCO3-rich Qatari dust. Carbonate forming plankton are absent in the water column. We propose that abiological, heterogeneous calcite precipitation (HCP) may be occurring. The mechanism is unknown but nucleation by CaCO3-rich Qatari dust may assist this process.

We explore the response of northeastern Pacific sea surface temperature (SST) to deglacial (similar to 16-7 ka) climate variability as recorded in U37K '-based SST reconstructions spanning 65 degrees N to 10 degrees S. Included in the analysis is a new 23 kyr SST record from core NH8P from the northwest Mexican Margin. We isolate spatiotemporal patterns in regional SSTs with trend empirical orthogonal function (TEOF) analysis. The dominant TEOF mode reflects deglacial warming associated with rising pCO2. Tropical and subtropical SSTs correlated most strongly with this mode, suggesting that the thermodynamic response of the tropical eastern Pacific to greenhouse gas forcing was the dominant driver of regional SST change during deglaciation. The second TEOF mode reflects millennial-scale variability and is most strongly expressed in subpolar SSTs. The synchronous timing between North Pacific and North Atlantic SST oscillations is evidence for the rapid transmission of millennial-scale climate perturbations between the basins, likely through an atmospheric teleconnection. SSTs at NH8P have no correlation with either leading TEOF mode as there is minimal change in SST at this site after similar to 20 ka. A model simulation of the LGM indicates that glacial cooling was muted in much of the Eastern Pacific Warm Pool (EPWP), in which NH8P lies, due to reductions in latent heat flux. This suggests that the wind-evaporation-SST feedback was responsible for the attenuation of EPWP cooling. Overall, this study highlights the distinct latitudinal trends in the Pacific's response to deglaciation.

We analyzed net-tow samples of natural assemblages of plankton, and associated particulate matter, from the Exclusive Economic Zone (EEZ) of Qatar in the Arabian Gulf. Size-fractionated suspended particles were collected using net tows with mesh sizes of 50 and 200 µm to examine the composition of small- and large-size plankton populations. Samples were collected in two different years (11 offshore sites in October 2012 and 6 nearshore sites in April 2014) to examine temporal and spatial variabilities. We calculated the excess metal concentrations by correcting the bulk composition for inputs from atmospheric dust using aluminum (Al) as a lithogenic tracer and the metal∕Al ratios for average Qatari dust. Atmospheric dust in Qatar is depleted in Al and enriched in calcium (Ca), in the form of calcium carbonate (CaCO3), relative to the global average Upper Continental Crust (UCC). To evaluate the fate of this carbonate fraction when dust particles enter seawater, we leached a subset of dust samples using an acetic acid–hydroxylamine hydrochloride (HAc–HyHCl) procedure that should solubilize CaCO3 minerals and associated elements. As expected, we found that Ca was removed in Qatari dust; however, the concentrations (ppm) for most elements actually increased after leaching because the reduction in sample mass resulting from the removal of CaCO3 by the leach was more important than the loss of metals solubilized by the leach. Because surface seawater is supersaturated with respect to CaCO3 and acid-soluble Ca is abundant in the particulate matter, we only used unleached dust for the lithogenic correction. Statistical analysis showed that for many elements the excess concentrations were indistinguishable from zero. This suggested that the concentrations of these elements in net-tow plankton samples were mostly of lithogenic (dust) origin. These elements include Al, Fe, Cr, Co, Mn, Ni, Pb, and Li. For several other elements (Cd, Cu, Mo, Zn, and Ca) the excess concentrations present after lithogenic correction are most likely of biogenic/anthropogenic origin. The excess concentrations, relative to average dust, for most elements (except Cd) decreased with distance from the shore, which may be due to differences in biology, currents, proximity to the coast, or interannual processes.

Alkalinity (Alk) and (dissolved inorganic carbon) DIC were measured on high resolution seawater samples, collected on November 2018 and May 2019 at seven stations in the Exclusive Economic Zone (EEZ) of Qatar. Calculated surface PCO2 averaged 472 matm in 2018 and 447 matm in 2019. Thus, the Arabian Gulf is degassing CO2 at present and will not take up atmospheric CO2 until 2042. Ocean acidification is not yet an issue in the EEZ of Qatar. The elevated PCO2 values are due to CaCO3 formation. Normalized NAlk and NDIC were calculated to remove the impact of increasing salinity. NAlk and NDIC decrease corresponding to a CaCO3/OrgC removal ratio of 2/1. We calculated the nitrate corrected and salinity normalized tracer, Alk*. Values of Alk* were negative, and the change in Alk* relative to Hormuz (DAlk*) indicated that there has been an average decrease of Alk* of -130 mmol kg-1. This decrease is due to CaCO3 formation but previous studies found no evidence for coccolithophorids. One obvious possibility is that Alk removal is due to CaCO3 formation in coral reefs. However, recent study of the composition of particulate matter found that the average particulate Ca concentration was 3.6%, and was easily acid soluble (Yigiterhan et al., 2018). These results suggest that a significant amount of particulate CaCO3 is present in the water column. One hypothesis is that the particulate Ca comes from carbonate rich atmospheric dust. Using Al as a tracer for dust and the average Ca/Al ratio in Qatari dust can only explain about 3% of the particulate Ca. An alternative hypothesis is that particulate CaCO3 may form in the water column due to abiological CaCO3 formation, as proposed recently for the Red Sea (Wurgaft et al., 2016). Precipitation of CaCO3 may be induced by the large inputs of nucleation sites in the form of atmospheric dust.

Fixed nitrogen is a limiting nutrient in many marine environments. Only a subset of the microbial community has the ability to fix dinitrogen gas (N-2). Here, we document the transcription of nitrogenase reductase subunit nifH in N-2-fixing bacteria in the dark suboxic and uppermost sulfidic layers of the northeast Black Sea. In shallower oxic waters, no N-2 fixation activity was detected using mRNA, and stable isotopes (delta N-15 and delta O-18) of nitrate supported the lack of N-2 fixation in oxic waters. On 2 expeditions in 2007, sampling in the suboxic zone (O-2 < 10 mu M, H2S below detection) and in deeper sulfidic waters yielded mRNA transcripts of nifH, even though NH4+ was 1-5 mu M. Multiple phylogenetic groups expressed nifH. Three uncultured groups of Cluster III type transcripts were detected, as well as 2 groups of Cluster I type sequences related to known sulfur oxidizers in the e-proteobacteria and Halorhodospira. The depth range where N-2 fixation was found was also the depth range of chemoautotrophic production, as determined by a maximum in suspended organic nitrogen concentrations and from 16S rRNA at these depths, which was dominated by known chemoautotrophs Sulfurimonas, SUP05, and BS-GSO2. We suggest chemoautotrophy and competition with chemoautotrophs for ammonium as reasons for N-2 fixation in the presence of ammonium. Profiles of N-2 gas unequivocally show the importance of N loss in the suboxic zone of the Black Sea; however, our data suggest a role for N-2 fixation. These results suggest that N cycling is seldom unidirectional.

N2 production by denitrification can occur in anoxic water or potentially inside organic particles. Here we compare data from the Black Sea, a permanently anoxic basin, during two organic matter regimes: suspended particulate organic matter concentrations were high in the oxycline after the spring bloom in March 2005 compared to lower organic matter concentrations in June 2005, May and October 2007, July 2008, and May 2001. For all cruises, N2 gas had a maximum in the suboxic zone (O2 < 10 μmol/L). During the high organic matter event (March 2005), an additional shallower N2 gas and δ15N‐N2 maxima occurred above the suboxic zone in the oxycline where oxygen concentrations were 30–50 μmol/L. Examination of 16S rRNA indicated that anammox bacteria were not present in the oxycline. The δ15N of biologically produced N2 in the oxycline in March 2005 was significantly enriched (+7‰ to +38‰), not depleted, as would be expected from water column fractionation. A simple diffusion calculation indicated that ammonium produced from remineralization inside particles could be oxidized to nitrate and then completely consumed by denitrification inside the particle. In this calculation, half of denitrified N atoms originated from organic N [δ15N = 11‰] and half of N atoms originated from ambient nitrate [δ15N = 5‰–7‰], producing enriched δ15N‐N2 values. We suggest that denitrifiers were active in microzones inside particulates in hypoxic waters above the suboxic zone of the Black Sea. Denitrification in particles may also explain previous data from the oxycline above ocean oxygen deficient zones.

The multilayered surface waters of the Black Sea contain aerobic, suboxic and anoxic layers that support both photoautotrophic (PP) and chemoautotrophic (ChP) biological production. During the R/V Knorr cruise from 15 to 25 April 2003, phytoplankton biomass (represented as chlorophyll-a), photoautotrophic and chemoautotrophic production (ChP) rates were determined in the southwestern Black Sea. Surface CM-a concentrations ranged from 0.06 and 0.62 ug/1 for the whole study area. These low values suggest post-bloom conditions. Integrated chlorophyll-a concentrations in the euphotic zone were as low as 2.4 mg m(-2) in the central gyre, while they were as high as 22 mg m(-2) in the Rim current. Integrated photoautotrophic production rates ranged from 70 to 664 mgC m(-2) d(-1). The lowest values were observed in the western central gyre and the highest values were found at the regions around Sakarya Canyon. Integrated ChP rates were 282 and 1013 mgC(-2) d(-1), which were equivalent to 45% and 83% of the overall water-column production for the central gyre and Rim current regions, respectively. Maximum rates are observed in the upper layer of the sulfide zone. The highest values of integrated ChP were in the vicinity of the Sakarya shelf and Rim current region. Extremely high values of ChP can occur in the Black Sea and appear to be driven by oxygen ventilation and redox cycling.

The Black Sea, the largest semienclosed anoxic basin on Earth, can be considered as an excellent natural laboratory for oxic and anoxic biogeochemical processes. The suboxic zone, a thin interface between oxic and anoxic waters, still remains poorly understood because it has been undersampled. This has led to alternative concepts regarding the underlying processes that create it. Existing hypotheses suggest that the interface originates either by isopycnal intrusions that introduce oxygen or the dynamics of manganese redox cycling that are associated with the sinking of particles or chemosynthetic bacteria. Here we reexamine these concepts using high‐resolution oxygen, sulfide, nitrate, and particle concentration profiles obtained with sensors deployed on profiling floats. Our results show an extremely stable structure in density space over the entire basin with the exception of areas near the Bosporus plume and in the southern areas dominated by coastal anticyclones. The absence of large‐scale horizontal intrusive signatures in the open‐sea supports a hypothesis prioritizing the role of biogeochemical processes.

The trace metal geochemistry of atmospheric dust and terrestrial surface particles were studied on the Qatar Peninsula from February 2014 to November 2015. We included samples of the mega dust-storm event on 01–02 April 2015. Atmospheric dust samples were collected using passive dust traps. Terrestrial surface deposits of recent dust accumulation and traffic particulate from roads were also sampled. All samples were total acid digested and analyzed for major and trace elements using ICP-OES analyzer. The concentration of thirteen elements (Ca, Mg, Ag, As, Cd, Cr, Cu, Mo, Ni, Se, Sn, Sr, Zn) were enriched in atmospheric dust samples, relative to upper continental crust (UCC). Calcium was especially enriched by up to 435% relative to UCC. About 33% of the total sample mass was CaCO3, reflecting the composition of surface rocks and soils in the source areas. Of the elements typically associated with anthropogenic activity, Ag, Ni, and Zn were most enriched relative to UCC, with enrichment factors (EF) of 182%, 233%, and 209%, respectively. Other metals, which normally reflect anthropogenic sources, including Pb and V, were not significantly enriched, with enrichment factors of 25% and 3%, respectively. Major elements (Al, Mn, Fe) were depleted (− 58%, − 35%, and − 5%, respectively) relative to UCC due to the large dilution effect of the enrichment of CaCO3. Back trajectories were determined at the date of sampling for each sample using the NOAA HYSPLIT model. These showed that the source of the dust particles was almost equally divided between northerly and southerly sources, except one sample, which appeared to originate from the west. More variability in particle source locations were observed during the winter months (October to March). Samples from the mega-dust storm were solubilized using an acetic acid-hydroxylamine hydrochloride leach procedure to obtain an upper estimate of the potential contribution of bioactive elements to surface seawater. The leach procedure solubilized a significant fraction of almost all elements. Ca was the element most affected (81% removed) because of the carbonate minerals present. Bioactive elements like Fe (25%) and P (58%) were also significantly solubilized. Because river input is so small to the Arabian Gulf, this solubilized fraction of dust is likely a major source of nutrients to surface seawater. Enrichment factors were also calculated with respect to the average composition of terrestrial surface deposits (TSD). Samples are not enriched significantly with respect to major components (EF < 2), with a depletion in Ca, K, Na in dust storm samples, reflecting a different origin. A significant enrichment of the same trace metals is evident in dust deposits and in traffic samples, but not in dust storms: Cu, Mo, Ni, Zn, possibly deriving from local atmospheric sources (traffic, industries). Samples with northern and southern origins were compared to see if the composition could be used to identify source. Only three elements were observed to be statistically different. Pb and Na were higher in samples from the south, while Cr was higher in those from the north.

Many of the Intergovernmental Panel on Climate Change’s Special Report for Emission Scenarios and Representative Concentration Pathways (RCP) projections (especially RCP 8.5 and 6) project CO2 emissions due to oil consumption from now to 2100 to be in the range of 32–57 Gb/yr (87–156 mb/d) or (195–349 EJ/yr). World oil production (crude plus condensate) was almost constant from 2002 to 2011 at about 74 ± 1 million barrels per day (mb/d) (US Energy Institute Agency, US EIA). There was an increase in world oil production after January 2011 that was mostly due to a surge of about 6 mb/d in light tight oil (LTO) production in the USA. This increased global oil production to just above 80 mb/d. Meanwhile, production in the rest of the world remained constant. The surge in the USA resulted in a sustained situation where supply was greater than demand globally, and this initiated a crash in the price of oil. The price of oil decreased from about $100 per barrel in mid-2014 to less than $30 per barrel in early 2016. Once the oil price declined, it was further enhanced and sustained by a decrease in demand due to a slowdown in the global economy. Because LTO is expensive to produce and was unprofitable after the price crash for the exploration and production companies, the surge in US production ended in about April 2015. Now, production of LTO in the USA is declining and global oil production is as well. New oil discoveries have reached a 70-year low, which does not bode well for future production. If the present patterns persist, it is unlikely that world oil production will exceed present US EIA oil production values of about 27–29 Gb/yr (equivalent to 75–80 mb/d) or (171–182 EJ/yr). It is unlikely that the demand for oil production required for CO2 emissions in RCP8.5 and RCP6 will be met.

We show that from 1955 to 2015, the inventory of oxygen in the Black Sea has decreased by 44 % and the basin-averaged oxygen penetration depth has decreased from 140 m in 1955 to 90 m in 2015, which is the shallowest annual value recorded during that period. The oxygenated Black Sea surface layer separates the world's largest reservoir of toxic hydrogen sulfide from the atmosphere. The threat of chemocline excursion events led to hot debates in the past decades arguing on the vertical stability of the Black Sea oxic/suboxic interface. In the 1970s and 1980s, when the Black Sea faced severe eutrophication, enhanced respiration rates reduced the thickness of the oxygenated layer. Re-increasing oxygen inventory in 1985–1995 supported arguments in favor of the stability of the oxic layer. Concomitant with a reduction of nutrient loads, it also supported the perception of a Black Sea recovering from eutrophication. More recently, atmospheric warming was shown to reduce the ventilation of the lower oxic layer by lowering cold intermediate layer (CIL) formation rates. The debate on the vertical migration of the oxic interface also addressed the natural spatial variability affecting Black Sea properties when expressed in terms of depth. Here we show that using isopycnal coordinates does not overcome the significant spatial variability of oxygen penetration depth. By considering this spatial variability, the analysis of a composite historical set of oxygen profiles evidenced a significant shoaling of the oxic layer, and showed that the transient "recovery" of the 1990s was mainly a result of increased CIL formation rates during that period. As both atmospheric warming and eutrophication are expected to increase in the near future, monitoring the dynamics of the Black Sea oxic layer is urgently required to assess the threat of further shoaling.

We present data on elemental concentrations of plankton net tow samples from the Exclusive Economic Zone (EEZ) of Qatar in the Arabian Gulf as part of a broader study of biogenic and lithogenic influences on particulate trace metal concentrations in the surface ocean. There are relatively few analyses of planktonic trace metals and their associated role in the biogeochemical system. We had the opportunity to investigate the composition of plankton in a region heavily affected by dust, a significant factor for phytoplankton growth. Our samples were collected from 2012 to 2015 using trace metal clean net tows with mesh sizes of 50 and 200 microns for measurements of phytoplankton and zooplankton, respectively. Samples were totally digested and analyzed by inductively coupled plasma-mass spectroscopy (ICP-MS). The biogenic portion was determined by subtracting the lithogenic portion from the total concentration. The lithogenic fraction was defined as the concentration of aluminum in the sample multiplied by a [Me]/Al ratio. Using average Qatari dust for these ratios generated a significant amount of overcorrection, so ratios were established using average continental crust (UCC). This method still caused some overcorrection for the lithogenic portion resulting in negative excess values for barium, molybdenum, and lead. These same elements showed the least consistency between measurements. For the other elements, a relative stoichiometry for plankton was determined as Fe > Cu ≈ Zn > As ≈ Cr ≈ Mn ≈ Ni ≈ V > Cd ≈ Co. We also found a significant near shore enrichment for 9 out of 13 elements analyzed, indicative of a possible influence of pollution. Acknowledgement This study was made possible by a grant from the Qatar National Research Fund under the National Priorities Research Program award number NPRP 6-1457-1-272. The abstract contents are solely the responsibility of the authors and do not necessarily represent the official views of the Qatar National Research Fund.

Atmospheric dust samples have been collected from different areas in Qatar and analyzed for major and trace element composition. This region of the Arabian Gulf represents an area largely affected by dust from natural and anthropogenic sources. Twenty one samples were collected during 2014 and 2015 from Al-Khor, Katara, Sealine, Al-Waab, and Qatar University by passive and active sampling techniques. Some bulk samples were collected during the massive megastorm that occurred in April 2015. Back trajectories were determined for each sample set using the NOAA HYSPLIT model over a 50 hour time interval. Samples were equally divided between northerly (n=12; northern Saudi Arabia, Kuwait or Iraq) and southerly (n=8; SE Saudi Arabia, United Arab Emirates and Oman) sources. One sample is expected to originate from westward, in Saudi Arabia. There is more variability noticed in source locations throughout the winter months (October to March), with more of them coming from the south (9 times) compared to summer months (April to September) for twice only. Dust samples were microwave-assisted, total acid digested (HF+HCl+HNO3) and oxidized with H2O2 before analyses were conducted by ICP-OES. Only 12 out of 23 elements (Al, Ca, Fe, K, Mg, Na, Ag, As, Ba, Be, Cd, Co, Cr, Cu, Li, Mn, Mo, Ni, Pb, Sr, V, Zn, P) of the Qatari dust samples were enriched relative to Upper Continental Crust (UCC). Calcium was especially enriched to a level of 400% relative to UCC. About 33% of the total sample mass was CaCO3, reflecting the composition of surface rocks in the source areas. Of the elements typically associated with anthropogenic activity, Ag, Ni, and Zn were the most enriched relative to UCC, with enrichment factors of 182%, 233%, and 209%, respectively. Other metals like Pb and V were not significantly enriched, with enrichment factors not exceeding 25% and 3%, respectively. Major elements (Al, Mn and Fe) were depleted relative to UCC due to the strong enrichment of CaCO3, with enrichment factors of -58%, -35% and -5%, respectively. Samples with northern and southern origins were separated to investigate if the elemental composition could be used to identify source/origin. Only three elements were observed to have a statistical difference. Pb and Na were higher in the samples collected from the south while Cr was higher in those from the north. This study aims to investigate present-day geochemistry of dust particles and its effect on the marine environment of the Arabian Gulf. The geochemical composition of dust is essential component for correcting lithogenic input to water column suspended matter samples. Acknowledgement This study was made possible by a grant from the Qatar National Research Fund under the National Priorities Research Program award number NPRP 6-1457-1-272. The abstract contents are solely the responsibility of the authors and do not necessarily represent the official views of the Qatar National Research Fund.