Carbonate Reef Research Articles

Facies Development of Carbonate Rock and System Tract Influence Based on Fullbore Formation Microimager (FMI) and Well Log Analysis on Carbonate Reservoir Tuban Formation, North East Java Basin

The North East Java Basin is known to be one of the basins that consist of Miocene carbonate rocks, like the reef carbonate of Tuban Formation. It has the potential hydrocarbons that can be explored. Therefore, the FMI log analysis is very important to identify carbonate rocks of Tuban Formation to know facies and characteristics of that carbonate rocks. The method used descriptive and analysis process of FMI and Gamma Ray log to determine facies of the carbonate rock and the system tract. Based on the result of FMI log analysis, there are variations lithofasies include mudstone, wackestone, packstone, grainstone, floatstone, rudstone and claystone. A collection of rock associations can interpret the reef facies. It involves back reef facies, reef core facies, and fore reef facies. The changes of lithofasies and reef facies are caused by sea level fluctuations and subsidence resulting in the system tract. The system tracts generated in the research area include transgressive and highstand system tract.

Zircon SHRIMP U-Pb dating of the Neogene coral reefs, Xisha Islands, South China Sea: implications for tectonic evolution

ABSTRACT The Xisha Block is a minor one in the South China Sea and an important tectonic unit in the northwestern part of the region. Zircon SHRIMP U-Pb ages for three volcanic intrusive core samples from Xike-1, an exploratory well penetrating the bioherms of the Xisha Islands. The core samples are from the Miocene reef carbonate bedrock and are recognized as dark-gray biotite-hornblende gabbro, gray fine-grained biotite diorite, and gray fine-grained granite, respectively. Zircon cathodoluminescence (CL) images and trace Th, U and Pb compositions of the zircons show that these rocks are of volcanic intrusive origin. Zircon SHRIMP U-Pb dating yielded six groups of ages, ranging from 2451-1857 Ma to early Cretaceous, which indicate that the formation and evolution of the Xisha Block was affected by the evolution and closure of Neotethys Ocean, probably within its eastern extension into South China Sea. Both old, deep-sourced material, including fragments from Rodina supercontinent, and recent mantle-derived magma products contributed to the emergence and formation of the Xisha block. The SHRIMP U-Pb results also proved that this process differed from that of the Kontum massif, the Hainan Block, and the South China Block, but is similar to that of the Nansha and Zhongsha blocks. The process was associated with the effects of Yanshanian magmatism induced by subduction mechanisms of the Paleo-Pacific Plate or the reworking of the multiple magmatisms since the Early to mid-Yanshanian, possibly jointly experienced by the Xisha-Zhongsha-Nansha Block.

Three new species and the molecular phylogeny of Antipathozoanthus from the Indo-Pacific Ocean (Anthozoa, Hexacorallia, Zoantharia).

In this study, three new species of macrocnemic zoantharians (Hexacorallia, Zoantharia) are described from localities in the Indo-Pacific Ocean including the Red Sea, the Maldives, Palau, and southern Japan: Antipathozoanthus obscurus sp. n., A. remengesaui sp. n., and A. cavernus sp. n. Although the genus Antipathozoanthus is currently restricted to species living on antipatharians, A. obscurus sp. n. is not associated with any living substrate and instead is found on coral reef carbonate substrate within narrow caves or cracks. The two new species that have association with antipatharians, A. remengesaui sp. n. and A. cavernus sp. n., can be distinguished by their relative coenenchyme development and the antipatharian species that each uses as substrate. Additionally, all new species described in this study have unique nuclear internal transcribed spacer region of ribosomal DNA (ITS-rDNA) sequences. Our results indicate that more phylogenetic studies focusing on increasing the numbers of species examined within each of the genera of Parazoanthidae are required in order to better understand the evolutionary history of substrate specificity within the family Parazoanthidae.

Exchange Dynamics Reveal Significant Accumulation of Dimethylated Sulfur by Mediterranean Benthic Communities

One fifth of Mediterranean waters can be classified as shelf – much higher than the global average. Consequently, the shelf / coastal zone plays a proportionally greater biogeochemical role than in the major oceans, including the support of a wide range of range of endemic or culturally important species and ecosystems. However, despite their known importance in regulating ecosystem function and the marine sulphur cycle, our understanding of the dynamics of dimethylated sulphur compounds such as dimethylsulphide (DMS) and dimethylsulphoniopropionate (DMSP) in Mediterranean benthic habitats is limited. Here, a community-level approach was adopted to quantify DMS and DMSP dynamics in Mediterranean ecosystems including seagrass (Posidonia oceanica) meadows, coralligene (an algal carbonate reef found along the Mediterranean shelf) and macroalgal stands. It was found that P. oceanica and coralligene are likely to act as significant benthic stocks of DMSP in the coastal / shelf environment. ‘Hotspots’ of water column DMS and DMSP processing were observed where net benthic production was high (e.g. P. oceanica meadows), demonstrating that benthic communities are able to modify DMS biogeochemistry in the overlying water column. High variability between, and within, habitat types illustrates the importance of ecosystem structure and light availability in determining benthic DMS and DMSP accumulation, and highlights a previously under-appreciated complexity in benthic dimethylated sulphur dynamics.

PRESENCE OF NATURAL GAS IN BASINAL FACIES OF THE ZECHSTEIN LIMESTONE IN A COPPER MINE IN SW POLAND

Zechstein (Upper Permian) dolomitic grainstones saturated with high pressure natural gas (including N2, CH4 and CO2) are present in the roof of workings in the Rudna copper mine in SW Poland. Natural gas commonly occurs in Zechstein reefal carbonates but this is the first occurrence of gas in basinal facies of the Zechstein Limestone in Poland. The reservoir dolomites are thin (ca. 1m), with porosity of 2–15% and permeability of less than a few mD. Pore size and pore distribution was analysed using microscope observations, X‐ray computed microtomography, mercury porosimetry and argon adsorption to investigate the dolomitic reservoir interval. The dolomites have variable reservoir properties, and stratigraphic traps with dimensions of several metres are present where high poro‐perm dolo‐grainstones are sealed by tighter dolomites. Geochemical data indicate that the methane and carbon dioxide present are derived from source rock intervals in the Kupferschiefer (Late Permian) and the Zechstein Limestone. The high content of nitrogen in the gas may be related to the thermal decomposition of Carboniferous organic matter. However, a biogenic origin for the nitrogen is also possible.

The short-term combined effects of temperature and organic matter enrichment on permeable coral reef carbonate sediment metabolism and dissolution

Abstract. Rates of gross primary production (GPP), respiration (R), and net calcification (Gnet) in coral reef sediments are expected to change in response to global warming (and the consequent increase in sea surface temperature) and coastal eutrophication (and the subsequent increase in the concentration of organic matter, OM, being filtered by permeable coral reef carbonate sediments). To date, no studies have examined the combined effect of seawater warming and OM enrichment on coral reef carbonate sediment metabolism and dissolution. This study used 22 h in situ benthic chamber incubations to examine the combined effect of temperature (T) and OM, in the form of coral mucus and phytodetritus, on GPP, R, and Gnet in the permeable coral reef carbonate sediments of Heron Island lagoon, Australia. Compared to control incubations, both warming (+2.4 °C) and OM increased R and GPP. Under warmed conditions, R (Q10 = 10.7) was enhanced to a greater extent than GPP (Q10 = 7.3), resulting in a shift to net heterotrophy and net dissolution. Under both phytodetritus and coral mucus treatments, GPP was enhanced to a greater extent than R, resulting in a net increase in GPP / R and Gnet. The combined effect of warming and OM enhanced R and GPP, but the net effect on GPP / R and Gnet was not significantly different from control incubations. These findings show that a shift to net heterotrophy and dissolution due to short-term increases in seawater warming may be countered by a net increase GPP / R and Gnet due to short-term increases in nutrient release from OM.

Sedimentary processes and depositional environments of the gas-bearing Horn River shale in British Columbia, Canada

The Horn River Basin in the northeastern British Columbia, Canada, is one of the largest unconventional gas-bearing basins in North America. The main reservoir of this gas accumulation is the Devonian Horn River Formation that is stratigraphically divided into three members, the Evie, Otterpark and Muskwa in ascending order. This study focuses on sedimentary processes and depositional environments of the Horn River Formation based on sedimentary facies analysis by the aid of well-log mineralogy (ECS) and total organic carbon (TOC) data from the Kiwigana well. The shales of the formation consist dominantly of silicate minerals (quartz, feldspar and mica) and subordinate clay mineral and carbonate materials, with TOC ranging from 0.3 to 7.6%. Three sedimentary facies were identified on the basis of centimeter-scale description of sedimentary structures and texture in borehole cores: homogeneous mudstone (HM), indistinctly laminated mudstone (ILM), and planar laminated mudstone (PLM). Integration of sedimentary facies, lithology and TOC suggests that the Horn River shale was primarily deposited in overall distal marine setting deeper than shelf or marginal slope, possibly base-of-slope to deeper basin plain off the carbonate reef (or shelf). Facies HM is siliceous and organic-rich, and dominant in the Evie and the overlying lowermost Otterpark members. It is interpreted as a pelagic to hemipelagic deposit formed mainly by suspension fall-out in an anoxic setting below maximum storm wave base. Likewise, facies ILM shows relatively high proportion of silicate minerals and TOC. This facies is identified frequently in the Muskwa and rarely in the Otterpark; it reflects a deposition from hemipelagic settling with an influence of persistent and weak bottom currents or nepheloid flows. Facies PLM, dominant in the large part of the Otterpark, is relatively depleted in silicate minerals and TOC. This facies indicates more frequent inflow of episodic turbidity currents punctuating the hemipelagic settling of the background sedimentation process. During the deposition of the Horn River Formation, the depositional site has experienced an earlier relative sea-level fall changing from a deep basin (Evie) to shallower marginal slope (middle Otterpark), and subsequent relative sea-level rise turning back to a deeper marine environment (Muskwa).

Seismic inversion of a carbonate buildup: A case study

The information within seismic data is band limited and angle limited. Together with the particular physics and geology of carbonate rocks, this imposes limitations on how accurately we can predict the presence of hydrocarbons in carbonates, map the top carbonate, and characterize the porosity distribution through seismic amplitude analysis. Using data for a carbonate reef from the Nam Con Son Basin, Vietnam, the expectations based on rock-physics analysis are that the presence of gas can be predicted only when the porosity at the top of the carbonate is extremely high ([Formula: see text]), but that a fluid contact is unlikely to be observed in the background of significant porosity variations. Mapping the top of the carbonate (except when the top carbonate porosities are low) or a fluid contact requires accurate estimates of changes in [Formula: see text]. The seismic data do not independently support such an accurate estimation of sharp changes in [Formula: see text]. The standard approach of introducing low-frequency models and applying rock-physics constraints during a simultaneous inversion does not resolve the problems: The results are heavily biased by the well control and the initial interpretation of the top carbonate and fluid contact. A facies-based inversion in which the elastic properties are restricted to values consistent with the facies predicted to be present removes the well bias, but it does not completely obviate the need for a reasonably accurate initial interpretation in terms of prior facies probability distributions. Prestack inversion improves the quality of the facies predictions compared with a poststack inversion.

Archaeological reconnaissance of the Late Pleistocene Red Sea coast in the Danakil

The paucity of archaeological evidence from the northern Afar Rift and Red Sea littoral remains a major hindrance to testing the hypothesised Out-of-Africa dispersal of early humans via the ‘Southern Route’—across the Bab al-Mandeb into Arabia and beyond. Encouraged by renewed geological studies (e.g. Atnafu et al. 2015), and recent archaeological discoveries (e.g. Walter et al. 2000; Beyin 2013), we initiated a pioneering exploration along the ancient Red Sea coast in the Danakil Depression (Figure 1a–c). The most conspicuous geomorphological features here constitute terraces of reef carbonates and associated marine deposits, formed when the depression was covered by the open Red Sea during the Middle to Late Pleistocene. Initial results of our exploration promise contexts in which the distribution and cultural/behavioural adaptation of early humans during the Out-of-Africa event can be closely investigated.

Archaeological reconnaissance of the Late Pleistocene Red Sea coast in the Danakil

The paucity of archaeological evidence from the northern Afar Rift and Red Sea littoral remains a major hindrance to testing the hypothesised Out-of-Africa dispersal of early humans via the ‘Southern Route’—across the Bab al-Mandeb into Arabia and beyond. Encouraged by renewed geological studies (e.g. Atnafu et al. 2015), and recent archaeological discoveries (e.g. Walter et al. 2000; Beyin 2013), we initiated a pioneering exploration along the ancient Red Sea coast in the Danakil Depression (Figure 1a–c). The most conspicuous geomorphological features here constitute terraces of reef carbonates and associated marine deposits, formed when the depression was covered by the open Red Sea during the Middle to Late Pleistocene. Initial results of our exploration promise contexts in which the distribution and cultural/behavioural adaptation of early humans during the Out-of-Africa event can be closely investigated.

Estimating regional coral reef calcium carbonate production from remotely sensed seafloor maps

Carbonate production on coral reefs is responsible for the provision of beach sands, for the maintenance of seawater chemical balances and for the growth of reef structure and associated habitat complexity. Key carbonate producers including hard coral, crustose coralline algae, foraminiferal sand and Halimeda were mapped from satellite imagery (spatial resolution 2.5m, mean overall accuracy=81%) and an upscaling model was applied to estimate carbonate production. A sensitivity analysis was conducted to evaluate the influence of employing different calcification rates for live coral on the upscaling model. Contemporary coral reef carbonate production for the 21 reef platforms of the Capricorn-Bunker Group (southern Great Barrier Reef) is estimated to be between 489,000 and 659,000t per year based on seawater chemistry, community composition, calcification rates and reef structural complexity (rugosity). The upscaling model was relatively insensitive to different parameterisations of live coral calcification employed, probably due to live coral being a relatively minor contributor by area (approximately 18% of total reef area throughout the study region). This suggests regional scale seafloor characteristics, such as percentage of area dominated by substrates prone to dissolution (e.g. coral rubble), have a strong bearing on calcium carbonate production and need to be given greater consideration The upscaling framework presented provides a new method for quantifying regional carbonate production that could be applied globally, and provides a valuable baseline against which future changes to carbonate production in this region can be assessed.

Stratigraphy of deformed Permian carbonate reefs in Saraburi Province, Thailand

Kilometre-scale thrusts affect the stratigraphic order of the sedimentary rocks that form the Khao Khwang Platform in central Thailand. The effect of these thrusts requires detailed analysis to unravel the original distribution of the sedimentary units and their associated facies. The Khao Khwang Platform comprises three carbonate-dominated units identified previously as the Phu Phe, Khao Khad and Khao Khwang Formations. These carbonates are intercalated with clastic, mixed siliciclastic and carbonate sequences: the Sap Bon, Pang Asok and Nong Pong Formations. The palaeogeography of the area prior to the Indosinian Orogeny is poorly known and has herein been investigated by combining structural reconstructions, microfacies and biostratigraphy in the Saraburi area. A stratigraphic model for the Yakhtashian–Midian carbonate platform palaeogeography using the well-developed exposed sections of the Khao Khwang Platform, located in the Saraburi Province of Thailand, is presented. This has revealed the presence of several separate carbonate platforms dominated by four major Middle Permian facies (peritidal, platform interior, algal reef and basin slope), which are dated using foraminifera and algae. This model integrates recent structural studies and considers their impact on the present-day distribution of the facies in terms of platform development using biostratigraphy and detailed microfacies analysis. Supplementary material: Characteristic images of the six facies, high-resolution stratigraphic logs, a timescale correlation chart and a table detailing the common fossils identified in this study are available at https://doi.org/10.6084/m9.figshare.c.3861796 .

Environmental drivers of coral reef carbonate production and bioerosion: a multi-scale analysis.

The resilience of coral reefs depends on the balance between reef growth and reef breakdown, and their responses to changing environmental conditions. Across the 2500-km Hawaiian Archipelago, we quantified rates of carbonate production, bioerosion, and net accretion at regional, island, site, and within-site spatial scales and tested how these rates respond to environmental conditions across different spatial scales. Overall, there were four major outcomes from this study: (1) bioerosion rates were generally higher in the populated Main Hawaiian Islands (MHI) than the remote, protected Northwestern Hawaiian Islands (NWHI), while carbonate production rates did not vary significantly between the two regions; (2) variability in carbonate production, bioerosion, and net accretion rates was greatest at the smallest within-reef spatial scale; (3) carbonate production and bioerosion rates were associated with distinct sets of environmental parameters; and (4) the strongest correlates of carbonate production, bioerosion, and net accretion rates were different between the MHI region and the NWHI region: in the MHI, the dominant correlates were percent cover of macroalgae and herbivorous fish biomass for carbonate production and bioerosion, respectively, whereas in the NWHI, the top correlates were total alkalinity and benthic cover. This study highlights the need to understand accretion and erosion processes as well as local environmental conditions to predict net coral reef responses to future environmental changes.

Altered sediment biota and lagoon habitat carbonate dynamics due to sea cucumber bioturbation in a high-pCO2 environment.

The effects of global change on biological systems and functioning are already measurable, but how ecological interactions are being altered is poorly understood. Ecosystem resilience is strengthened by ecological functionality, which depends on trophic interactions between key species and resilience generated through biogenic buffering. Climate-driven alterations to coral reef metabolism, structural complexity and biodiversity are well documented, but the feedbacks between ocean change and trophic interactions of non-coral invertebrates are understudied. Sea cucumbers, some of the largest benthic inhabitants of tropical lagoon systems, can influence diel changes in reef carbonate dynamics. Whether they have the potential to exacerbate or buffer ocean acidification over diel cycles depends on their relative production of total alkalinity (AT ) through the dissolution of ingested calcium carbonate (CaCO3 ) sediments and release of dissolved inorganic carbon (CT ) through respiration and trophic interactions. In this study, the potential for the sea cucumber, Stichopus herrmanni, a bêche-de-mer (fished) species listed as vulnerable to extinction, to buffer the impacts of ocean acidification on reef carbonate chemistry was investigated in lagoon sediment mesocosms across diel cycles. Stichopus herrmanni directly reduced the abundance of meiofauna and benthic primary producers through its deposit-feeding activity under present-day and near-future pCO2 . These changes in benthic community structure, as well as AT (sediment dissolution) and CT (respiration) production by S.herrmanni, played a significant role in modifying seawater carbonate dynamics night and day. This previously unappreciated role of tropical sea cucumbers, in support of ecosystem resilience in the face of global change, is an important consideration with respect to the bêche-de-mer trade to ensure sea cucumber populations are sustained in a future ocean.

Foraminifera from the Norian–Rhaetian reef carbonates of the Taurus Mountains (Saklıkent, Turkey)

AbstractNorian–Rhaetian reef carbonates are exposed in several localities in Taurus Mountains. They predominately contain hypercalcified sponges, followed by scleractinian corals and other less numerous organisms. A coherent Norian–Rhaetian reef structure is exposed near the small town of Saklıkent, west of Antalya. Foraminifers occur in reef carbonates of Saklıkent by numerous genera as shown in this paper. Two species —Siculocosta tauricaandSiculocosta sadati— are described as new. The foraminiferal association of Saklıkent is similar or almost identical to the associations known from the Norian–Rhaetian reefs of Sicily, Northern Calcareous Alps, and Greece but shows less similarity to the foraminiferal association from the Apennines, Italy. The most abundant foraminifers are milioliporoids, particularly galeanellids and cucurbitids. Some sessile and agglutinated foraminifers, includingAlpinophragmium perforatumFlügel, which mostly occurs abundantly in the Norian–Rhaetian reef carbonates, could not be found in the Saklıkent reef. This association of foraminifera is reported for the first time from a Norian–Rhaetian reef in the Taurus Mountains of Turkey.

Lithology, Rock Relief and Karstification of Minamidaito Island (Japan)

Minamidaito Island is a karstified coral island located on the Philippine plate. Karstification periods occurred during the end of the Pliocene, in major part of the Pleistocene, and in the Holocene. The first period of karstification can be observed in the lower Daito dolomitized limestone (5 Ma and 4 Ma); the karstification seems to have occurred as a result of sea level change caused by the cooler conditions of the paleoclimatic environment. Geological studies were performed to study reef carbonates in detail. Among them, biointrasparite limestone of framestone and bafflestone types with transitions to grainstone and dolomitized biointrasparry limestone of framestone and bafflestone types dominate. Calcimetric analyses established that in certain locations the distribution of limestone and dolomite differs from the distribution previously described. Rock relief reveals the unique formation of coastal karren, the development of the surface in the interior of the island, and the most characteristic periods of cave development.Key words: Minamidaito Island, lithlogy, coral dolomitized limestone, rock relief, coastal karren, karstification, vegetation.

Lessons Learned from CO2 Injection, Monitoring, and Modeling Across a Diverse Portfolio of Depleted Closed Carbonate Reef Oil Fields – The Midwest Regional Carbon Sequestration Partnership Experience

The MRCSP Michigan Basin Large-Scale Injection Project is evaluating CO2 storage and utilization in a complex of closed reservoirs at various stages of production and enhanced oil recovery (EOR) life-cycle. Since 2013, MRCSP has successfully injected and monitored the net storage of more than 600,000 metric tons of CO2 out of the goal of 1 million metric tons. A number of monitoring technologies have been tested, and various modeling approaches are being applied to evaluate CO2 migration, injectivity, capacity, and containment.

Palaeoenvironmental significance of Miocene larger benthic foraminifera from the Xisha Islands, South China Sea

We studied 344 samples from Well XK-1 in Xisha Islands, South China Sea, and identified 66 species of larger benthic foraminifera, providing critical evidence for biostratigraphy and palaeoenvironmental interpretation of the Miocene reef carbonate sequence. Three assemblages are recognized, namely, Spiroclypeus higginsi–Borelis pygmaeus Assemblage (Letter Stage Te5, Early Miocene, 1256.28–1180.15m), Nephrolepidina–Miogypsina Assemblage (Tf, Middle Miocene, 1031.10–577.04m), and Cycloclypeus–Heterostegina Assemblage (Tg, Late Miocene, 468.13–380.42m). On the basis of the palaeoecological preference of the larger foraminifera, we interpret that the Miocene carbonate sequence was deposited mainly in a warm tropical shallow water environment, characterized by five stages of continuous long-term evolution: backreef lagoon to shelf in the Early Miocene, normal to frontal reef in the early Middle Miocene, backreef lagoon to shelf in the later Middle Miocene, normal to frontal reef in the early Late Miocene, and proximal forereef shelf in the later Late Miocene.

Reef Habitat Type and Spatial Extent as Interacting Controls on Platform-Scale Carbonate Budgets

A coral reefs carbonate budget strongly influences reef structural complexity and net reef growth potential, and thus is increasingly recognised as a key “health” metric. Despite this, understanding of habitat specific budget states, how these scale across reef platforms, and our ability to quantify both framework and sediment production values remains limited. Here we use in-situ census data from an atoll rim reef platform in the central Maldives to quantify rates of both reef framework and sediment production and loss within different platform habitats, and then combine these data with high-resolution habitat maps to quantify contributions to platform wide carbonate budgets. The net reef framework budget for the entire platform is extremely low (0.12 G, where G = Kg CaCO3 m-2 yr-1), with a very high proportion (143,745 kg or 65.1%) of total framework production generated within the platform margin reef zones, despite these comprising only ~8% of platform area. Net platform-scale sediment budgets are higher (1.04 G), but most is produced in the reef and platform margin hardground habitats, of which ~80% derives from parrotfish bioerosion. Significant quantities of new sediment (up to ~1 G derived from the calcareous green algae Halimeda) are produced only in one habitat. All lagoonal habitats have negative or neutral net carbonate budgets. These data demonstrate the marked inter-habitat differences in reef carbonate budgets that occur across reef platforms, and the major dampening effect on overall platform scale budgets when rates are factored for habitat type and size. Furthermore, the data highlights the disproportionately important role that relatively small areas of reef habitat can have on the maintenance of net positive platform scale budgets. Because of the intrinsic link between carbonate production rates and reef-associated landform development and maintenance, these findings also have implications for understanding reef-associated landform stability. In this context the reef island at this site has been highly mobile over the last ~40 years, and we hypothesise that such instability may be being exacerbated by the measured low overall rates of framework and sediment generation.

New rates of Indian Ocean carbonate production by encrusting coral reef calcifiers: Periodic expansions following disturbance influence reef-building and recovery

Encrusting marine calcifying organisms, including coralline algae, bryozoans, calcareous worms, foraminifera and oysters, contribute to the geological development of reefs by precipitating calcium carbonate that binds detrital sediment and coral framework, and may produce new sediment after their death. However, it is difficult to quantify the relative importance of encrusting communities within contemporary reef carbonate production budgets because of a lack of regional datasets of either encrusting calcification or diversity, and the factors that control their settlement and growth. Here, we report rates of encrusting carbonate production for the Maldives in the Indian Ocean, determined by deploying artificial settlement substrates in shallow-water reef sites for 13months. Eight taxonomic groups are identified with high average rates of net calcification (mean=0.056±0.02gcm−2yr−1). Encrusting communities vary between exposed (upwards-facing) and cryptic (downwards-facing) reef habitats (p<0.05), driving inter-habitat differences in carbonate production rates (median cryptic=0.066gcm−2yr−1; median exposed=0.042gcm−2yr−1). These changes are a function of differences in light availability, grazing pressure, and wave exposure reflecting reversing monsoon conditions. Non-geniculate coralline algae are the dominant encrusting group (~50% cover) and major contributors to net calcification. The rapid expansion of coralline algae on post-mortem coral frameworks following the 2016 mass bleaching event in the Maldives has important implications for short-term carbonate cycling and reef budgetary states (pre-bleaching encrusting production=0.24kgm−2yr−1; post-bleaching=0.40kgm−2yr−1). Our findings suggest that encrusting communities add meaningful quantities of carbonate to reefs, and that their increased expression following declines in coral health could offset negative budgets on reefs where bioerosion rates are low, and aid future reef recovery by facilitating coral settlement.