Discovering Diverse Aragonite Crystal Forms in Abu Dhabi’s Coastal Lagoon

Recent research published in the Geological Society of America (GSA) Bulletin has unveiled a remarkable variety of aragonite crystal morphologies in the lagoons surrounding Abu Dhabi, United Arab Emirates. Aragonite, a metastable polymorph of calcium carbonate (CaCO₃), typically forms needle-like or fibrous structures in marine environments. However, this study challenges that conventional view by documenting hexagonal, tabular, chisel-shaped, and bladed varieties along environmental gradients from the shoreline to open coastal waters.

Abu Dhabi’s lagoons serve as a natural laboratory for studying modern carbonate sedimentation. These shallow, hypersaline bodies of water, bordered by sabkhas—flat, salt-encrusted plains—and fringed by mangroves, mimic conditions that produced vast ancient carbonate platforms. The findings highlight how subtle shifts in water chemistry and biology drive crystal diversity, offering fresh insights into the formation of hydrocarbon reservoirs that underpin the UAE’s economy.

The Unique Geology of Abu Dhabi Lagoons

The coastal lagoons of Abu Dhabi, part of the Arabian Gulf’s vast carbonate system, experience extreme environmental conditions. Salinities range from 40 to 70 parts per thousand (ppt), temperatures fluctuate between 20°C in winter and 35°C in summer, and water depths rarely exceed 5 meters. These factors promote rapid precipitation of aragonite as seawater becomes supersaturated with calcium and carbonate ions.

Sabkhas dominate the supratidal zone, where evaporation exceeds rainfall, leading to gypsum and halite precipitation. Seaward, lagoons transition to subtidal areas with ooid shoals and seagrass beds. This zonation creates ideal transects for observing mineralogical changes. Historically, studies focused on microbial mats and dolomitization in sabkhas, but less attention was paid to primary aragonite cements until now.

Methods Behind the Breakthrough Study

Researchers sampled sediments along multiple landward-to-seaward transects in the Abu Dhabi lagoon, from intertidal mudflats to outer shoals. Core samples and surface grabs were analyzed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) to characterize crystal habits, sizes, and compositions.

The transects spanned key zones: hypersaline inner lagoons (high salinity, warm), restricted middle lagoons (moderate flow), and open outer lagoons (normal marine salinity ~40 ppt). This systematic approach revealed patterns linked to physicochemical gradients, such as Mg/Ca ratios rising from 2 in open water to 5+ inland.

Newly Identified Crystal Morphologies

Near the shoreline, in high-salinity, Mg-rich waters, hexagonal prisms dominate, with flat faces and sharp edges up to 10 micrometers long. These give way to tabular plates in middle lagoons, resembling stacked pancakes 5-20 μm across.

Chisel-shaped crystals, with wedge-like terminations, appear in transitional zones, while needle-to-bladed forms prevail offshore where waters cool and Mg/Ca decreases. Bladed crystals can reach 50 μm, fanning out in radiating clusters. These morphologies were previously undocumented or rare in marine settings, expanding the known spectrum beyond fibrous needles.

• Hexagonal: Stable in evaporative, Mg-inhibited growth.
• Tabular: Favored by moderate supersaturation.
• Chisel: Transitional, influenced by organic templates.
• Bladed/needles: Open marine, uninhibited growth.

Such diversity suggests multiple nucleation mechanisms, including microbial mediation and inorganic templating by organic matter.

Environmental Controls Shaping Crystal Habits

Crystal morphology reflects local conditions. High Mg²⁺ inhibits calcite nucleation, favoring aragonite, but excess Mg slows growth, yielding blocky hexagons. Temperature inversely affects growth rate: warmer inner lagoons produce compact forms, cooler outer areas elongate blades.

Salinity drives supersaturation; organic matter from algae and bacteria provides nucleation sites. Step-by-step: Seawater ions cluster, Mg blocks calcite, aragonite nucleates on organics, growth rate dictates habit—slow for tabular/hexagonal, fast for needles.

In Abu Dhabi, monsoon-influenced flows flush outer lagoons, reducing salinity and promoting rapid growth. This mirrors global lagoons like Shark Bay, Australia, but Abu Dhabi’s aridity amplifies extremes. The full GSA Bulletin paper details these correlations.

Implications for Ancient Carbonate Interpretation

Aragonite transforms to calcite during burial diagenesis, preserving fabric but altering chemistry. Recognizing diverse precursors refines models of ancient platforms like the Permian Khuff Formation in UAE, a supergiant reservoir.

Hexagonal relics might indicate sabkha-like topsets; bladed suggest open shelves. This aids sequence stratigraphy, predicting porosity—tabular zones retain intercrystalline pores, enhancing permeability.

Real-world case: Thamama Group reservoirs (UAE) show neomorphic calcite after aragonite needles, informing enhanced recovery.

UAE’s Carbonate Reservoirs and Economic Stakes

The UAE holds 6% of global oil reserves, mostly in carbonate rocks. Abu Dhabi fields like Bu Hasa produce from aragonite precursors. Understanding modern analogs optimizes modeling, predicting diagenetic traps.

Statistics: ADNOC reports 97 billion barrels recoverable; carbonates contribute 80%. Crystal fabrics influence fracturing, flow units. Climate change raises sea levels, altering lagoons—research tracks shifts.

Stakeholders: ADNOC geologists, UAEU researchers collaborate on analogs. Khalifa University’s Earth Sciences drives such studies.

Role of UAE Universities in Carbonate Research

UAE institutions lead in applied geology. Khalifa University’s Petroleum Geoscience program analyzes sabkha analogs; UAE University’s Geosciences Department maps lagoon dynamics. NYU Abu Dhabi’s ACCESS center studies regional climate impacts on carbonates.

Funding from ADNOC, SRC supports fieldwork. Graduates fill reservoir engineering roles, boosting UAE’s knowledge economy. Recent collaborations with GSA highlight global standing.

Broader Global and Paleoclimate Insights

Beyond UAE, findings apply to Bahamas platforms, Shark Bay stromatolites. Paleoenvironmentally, diverse morphologies signal salinity gradients in rock record, reconstructing ancient oceans.

Future: Rising CO₂ acidifies oceans, favoring aragonite dissolution—Abu Dhabi monitors shifts. Actionable: Integrate morphologies in seismic models for exploration.

Challenges and Future Research Directions

Challenges: Rapid urbanization alters lagoons; climate shifts salinities. Solutions: Remote sensing, AI crystal analysis.

Outlook: Multi-proxy studies (isotopes, organics) decode biology vs. chemistry. UAE aims for net-zero by 2050; carbonate research aids carbon capture in reservoirs.

Photo by Juliana Malta on Unsplash

• Monitor pH drops impacting precipitation.
• Model diagenesis for CCS sites.
• Train geoscientists via UAE uni programs.

Stakeholder Perspectives and Impacts

Geologists praise expanded toolkit: “Transforms ancient rock interpretation,” per expert. ADNOC leverages for fields; academics publish in top journals.

For UAE youth: Earth sciences jobs surge 15% yearly. Explore at UAE academic opportunities.