Declining nutrient availability and metal pollution in the Red Sea

The study investigates how long-term human activities and climate change have altered the Red Sea’s chemistry and ecosystem functioning. The Red Sea is an oligotrophic, semi-enclosed basin whose nutrients primarily derive from vertical mixing in the north and wind-driven intrusion of nutrient-rich Indian Ocean waters in the south. Rapid regional warming, exceeding global ocean rates, has intensified stratification, potentially reducing nutrient availability and affecting biogeochemical cycles. Concurrently, anthropogenic pressures (e.g., shipping surge after the Suez Canal opening in 1869, regional oil development since 1938, coastal urbanization and industry) may have elevated trace metal inputs. The research aims to reconstruct multi-century variability of 15 elemental concentrations and accumulation rates from two open-sea sediment cores (north and south Red Sea), distinguish natural versus anthropogenic drivers, establish baselines for assessing human impacts, and inform mitigation policies for preserving Red Sea ecosystems.

Prior work shows the Red Sea has warmed rapidly (e.g., ~0.17 ± 0.07 °C per decade) with associated coral bleaching events and increasing stratification that can suppress nutrient supply. Nutrient inputs to the Red Sea rely on horizontal intrusion from the Indian Ocean via the Gulf of Aden and monsoon-modulated exchanges. Anthropogenic influences include trace metal pollution linked to regional industrialization, oil shipping/refining, and urbanization, with elevated metals reported in corals and zooplankton, especially in the north. Existing Red Sea sediment studies have often focused on coastal areas and lacked robust chronology, leaving open-ocean spatial and temporal variability of elements insufficiently resolved. This study addresses that gap using dated open-basin sediment cores and comprehensive elemental reconstructions.

• Study sites and sampling: Two open Red Sea sediment cores were collected in 2016 from >23 km offshore to minimize direct local anthropogenic influence: South Red Sea (17.33°N, 41.39°E; water depth 390 m; gravity corer; 20 cm length; 8.6 cm i.d.; Nov 2016) and North Red Sea (27.56°N, 35.28°E; water depth 1050 m; box corer; 18 cm length; 8.6 cm i.d.; Oct 2016). Cores were sectioned into 0.5 cm slices onboard, stored at -20 °C, freeze-dried, homogenized, and handled with clean protocols (gloves, clean/dirty hands SOP, acid-washed containers).
• Chronology: 210Pb activities (upper 0–10 cm) measured by alpha spectrometry after microwave acid digestion; 210Po measured assuming secular equilibrium. Ages were modeled using CF:CS and CRS models. Because 210Pb typically wanes by ~10 cm, deeper segments (>10 cm) were dated by AMS 14C (MICADAS), with dates calibrated using the Bacon Bayesian age–depth model in R. Blanks, replicates, and SRMs (e.g., IAEA-315) ensured quality control.
• TOC and TN: Determined with a CHNS analyzer (Flash 2000). About 10 mg samples in silver capsules underwent stepwise HCl acidification (3 M HCl, 60 °C) to remove carbonates; calibration standards showed R > 0.997. Blanks (capsules without sample) were run.
• Elemental analysis: ~50 mg sediment digested via microwave with HNO3/H2O2 and analyzed by ICP-OES (Agilent 5110) for Na, Mg, P, S, Ca and trace metals V, Cr, Fe, Ni, Cu, Zn, Cd; calibration curves (0.1–100 µg L−1) had R > 0.999; detection/quantitation limits <0.019/0.063 mg kg−1; QC included SRMs (NIST SRM 2702, 2703), CCVs, blanks, fortified blanks, QC standards, and spikes, with recoveries 90–110% and SRM agreement 78.91–119.57%.
• Silicon: Because nitric acid does not dissolve silicates and HF is hazardous for ICP-OES, Si was measured by X-ray fluorescence (HORIBA XGT-7000) on 12 representative sediment samples (four points per sample, 300 s scans).
• Accumulation rates: Element accumulation rates (EAR, µg cm−2 decade−1, dry weight) were computed as EAR = EC × dry bulk density × sedimentation rate (SR). SR was derived from depth–age relationships between bracketed dated horizons. Relative EAR changes before vs after the 1870s and EAR slope changes (linear regressions of EAR vs year) were calculated to evaluate temporal shifts.
• Statistics: Differences between north and south cores assessed with ANOVA or nonparametric tests (SPSS v27) depending on variance homogeneity. Linear regressions quantified EAR–time relationships and slope changes pre/post-1870. Pearson correlations among age, concentrations, and EARs were computed (OriginPro 2022). Significance threshold p < 0.05.

• Spatial patterns: South Red Sea sediments exhibited higher mean concentrations for most elements (e.g., TOC, TN, P, S, Fe, V, Zn, Ni, Cr, Cu, Cd) than the North, except Ca and Mg, which were higher in the North (Table 1). However, the North had higher element accumulation rates (EARs) for most elements.
• Nutrient source and gradients: Cd concentrations in southern sediments positively correlated with nutrient indicators (TOC, TN, P; R > 0.58, p < 0.01), consistent with higher nutrient levels and Cd in the south and supporting the Indian Ocean as the primary supplier of nutrients and trace metals.
• Post-1870 shifts in the South (declines): Mean EARs decreased after the 1870s for TOC (-14.9%), TN (-14.0%), P (-17.2%), S (-17.2%), Ca (-17.8%), Mg (-8.52%), Zn (-16.4%), Cr (-25.8%), Ni (-10.3%), Cu (-9.24%), Cd (-6.85%), and Cd (mass-based) -23.0%. Linear regression slopes of EAR vs year became more negative after 1870 (e.g., TOC slope decreased ~2.79×, TN ~2.93×, Cd ~5.54×), indicating accelerating declines. Reductions intensified after the 1970s.
• Post-1870 shifts in the North (increases): Mean EARs increased after the 1870s across all measured elements, with examples: TOC +30.1%, Na +25.4%, Si +12.4%, Mg +12.0%, Ca +8.29%, Fe +4.56%, V +13.8%, Cr +12.6%, Ni +19.5%, Cu +17.6%, Zn +9.29%, Cd +14.4%, TN +0.85%. EAR slope multipliers post-1870 vs pre-1870 were substantial for trace metals (V ×3.94, Cr ×8.00, Fe ×3.32, Cu ×10.03, Zn ×9.05, Cd ×4.10), and increases accelerated after the 1970s.
• Thermal proxy: Sediment Mg/Ca ratios increased with time in the South (R = 0.75, p < 0.01), indicating rising sea surface temperatures since the end of the Little Ice Age. The North showed pronounced changes in the last ~70 years, aligning with intensified local industrial activities and global change.
• Mechanistic interpretation: Declines in southern nutrient EARs likely reflect reduced nutrient inflow from the Indian Ocean due to warming-enhanced stratification and potentially weakened monsoon-driven exchanges. Northern increases in trace metal EARs align with intensified anthropogenic activities (shipping post-1869 Suez Canal opening; oil discovery 1938; industrialization and urbanization from the 1970s onward).
• Conceptual synthesis: The study introduces the Cai-Agusti Marine Crisis Conflux (CAMCC), describing the convergence of warming-driven thermal stress, declining nutrient availability, and escalating trace metal pollution in the Red Sea.

The reconstructed multi-century records from two open-basin sediment cores reveal a basin-scale divergence: the southern Red Sea shows declining nutrient-related EARs after the 1870s (accelerating after the 1970s), while the northern Red Sea exhibits increasing EARs for both major and trace elements over the same period. These patterns address the research questions by attributing southern declines primarily to physical-climate processes (regional and Indian Ocean warming intensifying stratification and potentially weakening monsoon-driven intrusions), and northern increases to anthropogenic sources associated with regional industrialization, shipping, and urbanization. The positive Cd–nutrient correlations in the south and higher southern trace metal concentrations support the Indian Ocean as a key source of both nutrients and trace elements. Rising Mg/Ca ratios corroborate sustained warming since the end of the Little Ice Age, with pronounced recent change in the north. The findings are significant for regional ecology: stratification-induced nutrient limitation can suppress primary production and reduce food availability, while trace metal enrichment, especially Cu and other pollutants, can impair coral physiology and reduce thermal tolerance, compounding bleaching risks. Given the Red Sea’s semi-enclosed nature and limited exchange, pollutants in the north may persist, increasing biotic exposure. The CAMCC framework highlights how concurrent thermal stress, nutrient depletion, and pollution likely interact synergistically with other stressors (e.g., UV-B, microplastics), threatening biodiversity, altering community structure, and elevating the risk of mass mortality and species loss. These insights provide a robust baseline for policy and management to mitigate local pollution sources and to address climate drivers.

This study establishes a high-resolution, dated baseline of elemental concentrations and accumulation rates from two open Red Sea sediment cores spanning ~500 years. Results show a post-1870 decline in nutrient-related EARs in the South Red Sea, consistent with warming-enhanced stratification limiting nutrient inflow from the Indian Ocean, and a contemporaneous rise in element and trace metal EARs in the North Red Sea linked to intensified anthropogenic activities, particularly after the 1970s. Increasing Mg/Ca ratios indicate sustained sea surface warming since the end of the Little Ice Age. Synthesizing these trends, the proposed Cai-Agusti Marine Crisis Conflux (CAMCC) encapsulates the co-occurring thermal stress, nutrient depletion, and pollution that imperil Red Sea ecosystems and may foreshadow global marine challenges. The authors call for urgent actions to reduce greenhouse gas emissions and adopt cleaner technologies to curb local pollution. These measures are essential to preserve Red Sea biodiversity and to prevent analogous crises elsewhere. Future research could extend spatial coverage with additional dated cores, refine source apportionment of trace metals, and integrate physical-biogeochemical modeling to project ecosystem trajectories under varying climate and mitigation scenarios.

• Geographic and sampling scope: Only two open-sea sediment cores (one south, one north) were analyzed, which may limit spatial generalization across the basin’s diverse subregions.
• Coring methods: Different corers were used (gravity vs box corer) due to logistical constraints, which could introduce methodological heterogeneity, though clean handling protocols were applied.
• Chronology constraints: 210Pb dating is limited to the upper ~10 cm; deeper chronology relies on 14C, which requires calibration and may have inherent uncertainties. 14C is unsuitable for recent surficial layers due to bomb 14C, necessitating reliance on 210Pb for the upper sections.
• Silicon measurements: Si concentrations were obtained by XRF on a subset of 12 samples, potentially constraining temporal resolution for Si compared with other elements.
• Proxy interpretation: Mg/Ca ratios in bulk sediments are influenced by mineralogical composition and diagenesis; while trends align with warming, non-thermal factors could contribute.
• Source attribution: While temporal correlations align with industrialization and shipping, direct source apportionment (e.g., isotopic tracers) was not conducted, leaving some uncertainty in precise pollutant sources.