Abiotic and Biotic Processes Controlling Deposition of Calcite and Hydrotalcite Calcretes on Niue Island, Southwest Pacific

Abstract

Calcretes are indurated terrestrial carbonates that are widespread in arid and semi-arid settings and serve as important archives of present and past environments. Here, we use geochemical tools to explore the nature and origin of calcretes documented from tropical Niue Island in the Southwest Pacific. The study recognizes two types of calcretes that differ in their mineral assemblage, microfabrics, elemental chemistry, and carbon and oxygen isotopes. The calcretes common in the paleo-lagoon soils consist of 90% low-Mg calcite and ~10% highly weathered Mg-Al silicates. These pedogenic calcretes formed in the soil profiles within the vadose zone bear the following distinctions: (i) Fe/Al ratio of 0.75, identical to the ratio in soils (Fe/Al = 0.76 ± 0.5), substantiating the link between the calcretes and soils; (ii) presence of rhizoliths, root voids, micritic nodules, and clasts, which are consistent with a pedogenic calcrete fabric; and (iii) ¹³C and ¹⁸O depletions of −10.6‰ and −5.3‰, respectively, which are compatible with carbon sources from microbial and root respiration, as well as formation in oxygen isotope equilibrium with vadose waters. Unlike the pedogenic calcrete, a rare calcrete from the coastal terrace contains an exceptionally rare hydrotalcite [Mg₆Al₂(CO₃)(OH)₁₆(H₂O)₄] mineral (65%) coated by microbial films. We contend that the hydrotalcite-rich calcrete was deposited through interaction of dolomite with seawater, similar to the method of producing hydrotalcite in the laboratory. ¹³C and ¹⁸O enrichments of 0.8 to 1.7‰ and −1.0 to −1.6‰, respectively, are in agreement with (i) mixed carbon sources consisting of microbial CO₂ degassing, seawater HCO₃, and dolomite dissolution, and (ii) oxygen isotope equilibration with seawater-derived fluid.

1. Introduction

Two types of carbonate-rich calcretes have been documented from Niue Island in the Southwest Pacific (Figure 1). One is commonly associated with the island’s interior soils (Figure 2A) [1]. The other, confined to the coastal terrace, is uncommon and has been described from field observations as a “Palagonite tuff” [2,3], and an “Iron-rich deposit (Figure 2B) [4]. Neither one of those studies substantiated the field observations with laboratory investigations.

Here, we report the results of a laboratory investigation of the two contrasting calcrete types occurring in Niue Island. The calcrete block exteriors conceal their content (Figure 2A,B), and therefore, examination in the field offers no clues as to their origin or provenance. In this study, we employ X-ray Diffraction (XRD) to identify the calcrete mineral constituents and petrographic microscopy of thin sections to unravel the microfabrics of the minerals and organic residues. Additionally, we acquired elemental chemistry and stable carbon and oxygen isotopes from hand-size subsamples to discern their origin and provenance. In conjunction, the results allowed us to deduce the factors controlling deposition in a sedimentary setting atypical of calcrete formation [5,6].

2. Niue Island Setting

Niue Island is an uplifted former atoll in the tropical Southwest Pacific and, at 259 km², one of the largest carbonate islands in the region (Figure 1). The geology and landscape of the island have been shaped primarily by (i) early subsidence of the underlying extinct volcano during the Early-Mid Miocene to Pliocene [7,8,9]; (ii) uplift since the early Pleistocene resulting from upwarping of the lithospheric plate as it traveled toward the Tonga Trench [10]; (iii) sea-level fluctuations since the Late-Miocene time [11,12]; and (iv) enhanced sediment transport and erosion associated with the sea-level cycles and frequent cyclones [13,14]. The depositional history of the carbonate sediments in Niue, comprising limestone, dolostone, and remnant coral aragonite, has been investigated in outcrops, quarries [4,7,13,14,15], and cores up to 320 m long [9,11,16].

The island’s geology is differentiated into a modern to mid-Holocene age shore platform at sea-level [13], a prominent Alofi Terrace of the Late Pleistocene age rising to an elevation of about 20–23 m asl, and a Late Miocene to Pliocene-age Mutalau Atoll consisting of a lagoon enclosed by a ridge that preserves the former coral atoll topography [7].

Niue is blanketed by a thin veneer of ~35 cm thick, highly weathered soils dominated by Fe and Al minerals consisting of goethite, gibbsite, and boehmite [1]. The soils have been systematically sampled and investigated in order to explain the origin of their unusually high radioactivity [3,17,18], among others. The Alofi Terrace and the seaward slopes are mantled by tropical black earths [19]. The soils on the remainder of the island are tropical terra rossa that are low in silica and high in iron oxide and alumina. Lithified red to light-brown colored terra rossa soil filling sinkholes in the carbonate sands and overlying massive dolostone is commonly observed in the former lagoon (Figure 3). The provenance of the highly weathered Niuean soils has long been a subject of debate, bearing on three explicit sources: (i) volcanic ash and meteoric dust [17], limestone from the carbonate platform [18], and (iii) pumice rafts [1].

The occurrence of odd, solitary rocks underlying soils and overlying sediments on Niue has long been known to the locals, who nicknamed the more common yellowish-gray as “Brown Rock” (Figure 2A) and the scanty reddish-brown as “Black Rock” (Figure 2B). [2]. A solitary “Black Rock” found on a shelf ~3 m asl on the northwest side of Niue was described by [2] (pp. 818–819) as an “oxidized, yellow palagonite-cemented tuff with a hard, thick dark-brown crust containing much CaCO₃ and appearing slightly permeable”. Neither the collected sample nor the field location have been ever found [3]. More recently, [4] reported of another “Black Rock” type the authors examined on the southern coast near Vaiea (Figure 1). This “Black Rock”, about 1.1 m in length, projects from the edge of the terrace at ~3.2 m asl and is frequently engulfed by the waves (Figure 2B). These authors [4] identified the rock as an “iron-rich deposit” formed by groundwater in cracks within the limestone platform. The survival of the rock in a high-energy environment was explained by its higher resistance to erosion than the surrounding limestone [4]. Unlike the scarcity of the “Black Rock”, the distribution of the rectangular, flat “Brown Rock” is geographically wider and commonly occurs underlying the soil blanketing the ancient Mutalau barrier reef (i.e., “makatea”) and lagoon [1] (Figure 2A).

A “Brown Rock” from the top of the Mutalau reef and a “Black Rock” from the coast near Vaiea were investigated in the field, and representative subsamples were taken for further studies (Figure 1). Field observations identified the rocks as calcretes [1]. In the following, we report on the laboratory methods we employed and the analytical results rendering data on the mineral constituents, textural relation between components, elemental chemistry, and carbon and oxygen isotopes that have a bearing on the origin and provenance of the calcretes.

3. Methods and Results

3.1. Mineral Compositions

Several grams of representative rock samples were powdered in an agate mortar, thoroughly mixed, and bottled in sealed vials. The mineralogy of the randomly oriented powders was determined by X-ray diffraction (XRD) using a benchtop Bruker X-ray diffractometer. XRD data were collected with Cu Kα radiation by step-scans at 0.04°/2θ increments with a fixed counting time of 1.0 s. Automated quantitative calibration of mineral weight percentages was based on integrated peak intensity procedures.

The XRD scans, shown in Figure 4A,B, exhibit major dissimilarities between the calcretes. The mineralogy of the “brown rock” (henceforth, PC-C) presents no surprises. It is dominated by low-magnesium calcite at ~90% abundance, followed by a small but significant assemblage of Mg-Al silicates (goethite and gibbsite) and a hydrated Ca-Al phosphate [CaAl₃(PO₄)(PO₃OH)(OH)₆] amounting to ~10% abundance. The peaks of gibbsite- and crandallite-related minerals carry a significant uncertainty arising from the overshadowing of the dominant calcite peaks. In contrast, the dominant carbonate minerals in the “Black Rock” (henceforth, HT-C) are hydrotalcite and magnesian–calcite at 65% and 30% abundance, respectively. Traces of goethite are present in the XRD scan (Figure 4B). The carbonate minerals of powder aliquots from the two calcretes were preferentially dissolved with dilute HCl and filtered in order to examine the insoluble residue (IR). Concentration of the IRs by a magnet showed the presence of abundant magnetite clusters in the PC-C, but total absence in the HT-C.

Hydrotalcite is a layered double Mg and Al hydroxy-carbonate hydrate (LDH) represented by the general formula [Mg₆Al₂(CO₃)(OH)₁₆(H2O)₄] [20]. It is a rare natural mineral and is commonly found in association with weathered ultramafic and serpentinite rocks [21,22,23,24]. With the two exceptions below, hydrotalcite occurrences are exceptionally rare in sedimentary rocks. The exceptions are the occurrences of HT in high-Mg mudstones from the Kozani Basin in Greece [25] and a sparse distribution in lithified terra rossa soils from the Cayman Islands [26].

3.2. Microfabrics

Polished thin sections were prepared from the PC-C and HT-C and examined under a Nikon SMZ800 petrographic microscope using reflected light under a range of magnifications.

The most outstanding feature that clues us in on the pedogenic origin of PC-C is the presence of organic matter-infilling voids representing decayed root mats (Figure 5A,B). The root voids have locally evolved into rhizoliths (calcified roots) as displayed in Figure 5A,B,D. The presence of brown-micritic nodules, micrite screens around the root voids, mottled-gray calcites, and calcite clasts (Figure 5C) points to pedogenic calcrete fabrics [27,28]. Not unlike the PC-C, the HT-C also displays microfabrics typical of calcretes (Figure 5A,B). In contrast to PC-C, which exhibits high-porosity and immature rhizoliths, the HT-C is more compacted and shows lower porosity, attributed to infilling of the rhizoliths with hydrotalcite coated by microbial films (Figure 5A). Occasional dissolution voids are observed in Figure 5B. The small snail shell that found shelter in the void is likely modern. The PC-C exhibits “immature” fabrics by comparison with the HT-C’s “mature” fabrics. On this basis, we surmise that the former may be of a younger age than the latter.

3.3. Elemental Chemistry

The major oxide composition of PC-C and HT-C was determined by XRF on powdered samples prepared as fused disks, and the results are listed in

Table 1. Elemental chemistry of selected elements in weight % in calcretes. Niue soils (A-horizon) data are from [18] and list mean and SD values; n = number of analyzed soil samples.

Sample	SiO2	Al2O3	Fe2O3	MgO	CaO	MgO/Al2O3	Fe2O3/Al2O3
PC-C	1.5	12.3	9.3	0.6	36.5	0.05	0.75
HT-C	0.4	10.9	8.0	17.9	21.1	1.6	0.73
Niue Soils	0.82 ± 0.6	23.5 ± 10.8	17.9 ± 8.6	0.9 ± 2.0	10.4 ± 11.8	0.04 ± 0.1	0.76 ± 0.5
n	(n = 89)	(n = 107)	(n = 89)	(n = 94)	(n = 115)

.

Silica is ~3.7 times higher in PC-C relative to HT-C, whereas the latter has ~30 times more Mg than the former (Table 1). The discrepancies are attributed to a higher abundance of detrital silica-rich minerals inherited from the soils in PC-C, and the Mg enrichment in HT-C is explained by the dominant presence of Mg-rich hydrotalcite and magnesian calcite.

A weathering index (WI) developed by [1] serves as a useful indicator of the preferential silica removal during weathering relative to the sum of iron and aluminum concentrations: (SiO₂/(Al₂O₃ + Fe₂O₃)). Both calcrete types exhibit exceptionally low WIs values of 0.069 and 0.021, suggesting a high degree of weathering that compares well with the mean WI of 0.02 of the highly weathered Niue soils (Table 1).

3.4. Carbon and Oxygen Isotopes

Well-mixed, powdered aliquots of ~1.0 mg by weight of PC-C and HT-C were reacted with 100% orthophosphoric acid at 50 °C. The carbon and oxygen isotope values of the evolved and purified CO₂ were determined in a continuous flow mode (CF) using a Gasbench coupled to a modified Delta-Plus Isotope Ratio Mass Spectrometer (CF-IRMS). The ¹³C/¹²C and ¹⁸O/¹⁶O ratios are reported in the conventional delta (δ) notation in per mil (‰) relative to the Vienna-Pee Dee Belemnite (V-PDB). The analytical precision for both oxygen and carbon based on standard and sample repeats is ±0.1‰ (1σ). The water δ ¹⁸O values listed below are reported on the conventional V-SMOW scale. Conversion from the V-PDB to V-SMOW scale is given by the following equation [29]:

δ ¹⁸O (V-PDB) = 0.97001 × δ ¹⁸O (V-SMOW) − 29.99

(1)

Because the NBS-19 standard is a calcite, no kinetic fractionation factor correction associated with the acid reaction to liberate CO₂ was necessary for δ ¹⁸O of calcite-dominated PC-C samples. In contrast, the kinetic fractionation factor correction for hydrotalcite-dominated HT-C samples is unknown, and hence, no correction to the δ ¹⁸O values was performed. The total range of the correction factor for a wide range of carbonates is only 1‰, and therefore, ignoring it should introduce only a small uncertainty in δ ¹⁸O values of hydrotalcite, not exceeding several tenths of a permil [21].

The δ ¹³C and δ ¹⁸O composition of the paired calcrete subsamples are listed in

Table 2. Carbon and oxygen isotopes of paired subsamples from the calcite-rich (PC-C) and hydrotalcite-rich (HT-C) calcretes in Niue Island. Note the contrasting isotope compositions between the two calcrete types.

Sample	δ 13C (‰ V-PDB)	δ 18O (‰ V-PDB)	δ 18O (‰ V-SMOW)
PC-C (1)	−10.7	−5.4	25.4
PC-C (2)	−10.5	−5.2	25.6
HT-C (1)	0.8	−1.6	29.3
HT-C (2)	1.7	−1.0	29.9

. Significant isotope differences observed between the PC-C and HT-C can be summarized as follows. PC-C exhibits substantially greater ¹³C and ¹⁸O depletions (−10.5 to −10.7‰ V-PDB and −5.2 to −5.4‰ V-PDB, respectively) relative to the HT-C (0.8 to 1.7‰ and −1.0 to −1.6‰ V-PDB, respectively).

The ambient conditions conducive to the deposition of PC-C can be established on the basis of the δ ¹³C and δ ¹⁸O values because equilibrium isotope fractionations between calcite and dissolved inorganic carbon (DIC) and between calcite and H₂O are well known [30,31]. We contend that the PC-C calcrete was formed in isotope equilibrium with the soil fluids on the basis of the following considerations. The measured δ ¹³C values of DIC from Niue are −11.7 ± 2.3‰ V-PDB (n = 6) in drips from caves representing water in the vadose zone, and −9.1 ± 2.7‰ V-PDB (n = 18) in groundwater [32]. Assuming an ambient soil temperature of 25 °C, which is the annual average air temperature in Niue, the predicted δ ¹³C composition of a calcite in isotope equilibrium with the DIC would be −8.2 to −10.8‰V-PDB, which is, given the uncertainties, in good agreement with the measured δ ¹³C values in PC-C (Table 2). On the basis of a radiocarbon material mass balance, [32] estimated that up to 91% of the carbon in the vadose zone is derived from soil CO₂ microbial respiration (δ ¹³C = −29.4‰ V-PDB), and the remaining 9% from dissolution of the underlying limestone/dolostone (δ ¹³C = −0.4 ± 0.9‰ V-PDB, n = 149).

Drips from tips of stalactites in coastal caves and groundwater wells on Niue yield δ ¹⁸O compositions of −4.5 ± 0.1‰ V-SMOW (n = 5) and −4.5 ± 0.14‰ V-SMOW (n = 18), respectively, representing the mean values of annual rainfall [32]. Using the temperature-dependent CaCO₃-H₂O experimental equation of [31], a predicted calcite deposited in isotope equilibrium with the Niuean waters at the ambient annual temperature of 25 °C would yield a δ ¹⁸O of −7.1‰ V-PDB. The predicted value is roughly close to the measured δ ¹⁸O values of the paired PC-C samples (Table 2), considering the likely effect of slight evaporation of the soil fluids that will cause ¹⁸O-enrichment in the water, as well as the uncertainty of soil temperature.

Reconstruction of the ambient conditions conducive to hydrotalcite deposition from carbon and oxygen isotopes is complicated. This is because, with the singular exception below, no studies have reported stable C and O isotope data for pure hydrotalcite or samples containing hydrotalcite in sedimentary rocks. Exceptional are the δ ¹³C and δ ¹⁸O reports of one hydrotalcite associated with serpentinite [21] and hydrotalcite mineralization of ultramafic mine tailings [23,24]. Importantly, the isotope fractionations, either equilibrium or kinetic, during hydrotalcite uptake of labile carbonate ions, structural OH groups, and interlayer H₂O molecules are unknown. However, given the exposure of the HT-C to atmosphere and seawater (Figure 2B) and the association with the Niuean dolomite as a principal source of Mg, we conjecture that the ¹³C and ¹⁸O enrichments in the hydrotalcite (Table 2) were controlled by mixing of three sources (i) atmospheric CO₂ (δ ¹³C = −8.5 ± 1‰, [33]; (ii) HCO₃ source in dolomite dissolution (δ ¹³C and δ ¹⁸O of 1.2 to 2.8‰ and 2.8 to 4.2‰, respectively) [16]; and (iii) structural OH and interlayer H₂O of seawater provenance (seawater δ ¹⁸O = −0.15‰ V-SMOW, [32].

4. Discussion

Pedogenic calcretes composed of low-Mg calcite are terrestrial deposits of widespread occurrence that form in soil profiles within the vadose zone [5,6]. Calcrete-bearing soils are most common in arid and semi-arid continental settings where the annual rainfall amount is <760 mm and where evaporation exceeds rainfall (e.g., in the Mediterranean climate), causing moist deficits [6]. Niue Island, located in the tropics, receives an annual rainfall of 2036 ± 538 mm (mean and SD over a 92-year period [34]), and as such, it seems to be an atypical setting for calcrete formation. In the following, we discuss the abiotic and biotic factors involved in the formation of the two contrasting PC-C and HT-C calcretes documented in this study.

4.1. The Rainfall Factor

Rainfall on Niue Island is highly variable on inter-seasonal and inter-annual time scales [32,34]. The inter-seasonal variability is driven by the South Pacific Convergent Zone (SPCZ) that moves seasonally over Niue as an appendix of the Intertropical Convergence Zone (ITCZ). It results in a monsoon season from December to April with a mean monthly rainfall of 307 mm and an average monthly air temperature of 26 °C, and a dry season from May to November with a mean monthly rainfall of 84 mm and a mean air temperature of 24 °C. The El Niño–Southern Oscillation (ENSO) phase changes with 4.3 to 6.0 years of periodicity and exerts a dominant role on rainfall variability on inter-annual intervals. Severe droughts are associated with El Niño events [32]. The regularity of rainfall switches is interrupted periodically by powerful cyclones accompanied by torrential rainfall. Twelve tropical cyclones struck Niue in the time intervals of 1970–1975 and 1980–1985 [34]. Importantly, there is no surface drainage on the island, and consequently, rapid rainfall infiltration occurs as a result of the high porosity of the carbonate sediments. It can be concluded, therefore, that large annual rainfall values are concealing the droughts on inter-seasonal and inter-annual time scales that are conducive to calcrete formation in Niue.

Calcretes are notoriously difficult materials to be absolute-dated by either radiocarbon or Uranium Series assays. The difficulties arise from problems of alteration, contamination with detrital mineral phases, and open system behavior [5,6]. Presently, the absolute ages of the calcretes studied here are unknown. However, on the basis of the observed excellent preservation of the organic matter and the pristine rhizolith structures (Figure 5 and Figure 6), we surmise that the absolute age of the calcretes is likely not older than the mid-to-late Holocene. If so, the question arises as to whether or not the extant rainfall factor promoting deposition of calcretes in Niue during drought periods was also active in the past. A recent paleohydrological multi-proxy stalagmite record from a Niue cave offers evidence that ENSO–controlled SPCZ variability during the mid-to-late Holocene was not different than today [35].

4.2. Abiotic and Biotic Factors

The data presented in the preceding section suggest that both abiotic and biotic processes are controlling the deposition of the PC-C calcretes. These are (i) excess evaporation over rainfall during droughts; (ii) dissolution of the carbonates and leaching of Ca in the vadose zone; (iii) invasion of atmospheric CO₂; (iv) microbial and root respiration in the soil rhizosphere controlling the pCO₂; (v) CO₂ degassing; and (vi) elevated Ca and HCO₃ solutes from biogenic and abiotic sources giving rise to supersaturation and precipitation of low-Mg calcite.

The data acquired from the HT-C contradict previous identifications as a palagonite tuff [2,3] or iron-rich rock [4] and establish pertinence to an exceptionally rare class of calcretes not previously acknowledged that are dominated by hydrotalcite. The hydrotalcite abundance in the HT-C calcrete raises questions on the source of Mg and the factors controlling its deposition. Association of hydrotalcite mineralization with serpentinite and ultramafic rocks and experimental studies have prompted the suggestion that Mg released in the groundwater by weathering and leaching of Mg-silicates (e.g., olivine) and/or Mg-hydroxides (e.g., brucite) reacts with atmospheric-derived CO₂ to produce layered double hydroxides (LDH) [23,24,36]. The absence of ultramafic rocks in the Niue shallow subsurface makes such a Mg source unattainable. Here, we propose that the Mg source is in the abundant dolomite underlying the Niuean soils (Figure 3).

On the basis of our observations and analytical data, we propose the following model of HT-C calcrete formation: (i) erosion and transport of MgO-rich terra rossa soil from the nearest Mutalau crest to the terraced coast; (ii) dolomite dissolution and CO₂ uptake from atmospheric and seawater sources; and (iii) induration of the terra rossa soil with hydrotalcite and Mg-calcite. Our proposed HT-C depositional model is supported by the following arguments:

(i)

Niue Island has a high potential for sediment erosion and transport [13]. The highly dynamic environment will account for the transport of terra rossa (Figure 3) to the coast (Figure 2B).

(ii)

Dolomite serves as the principal source of Mg.

(iii)

The ¹⁸O-enrichment exhibited by the hydrotalcite + Mg-calcite minerals is consistent with deposition from seawater whose waves regularly flood the calcrete.

(iv)

The ¹³C-enrichment is compatible with mixed carbon sources from microbial respiration, kinetic invasion of atmospheric CO₂, HCO₃ from seawater, and CaMg(CO₃)₂ dissolution. Our proposed model of hydrotalcite precipitation by dolomite–seawater interaction is supported by laboratory synthesis of hydrotalcite via the reaction of dolomite with seawater [37].

5. Summary

(i)

Droughts occurring with inter-seasonal and inter-annual frequencies frame the uplifted former atoll of Niue Island in the Southwest Pacific as a suitable setting for calcrete deposition.

(ii)

Two types of calcretes have been documented on the island: (i) a “common” calcite-rich calcrete associated with the soils in the paleo-lagoon and (ii) a unique hydrotalcite-rich calcrete occurring on the coastal terrace and bathed by high waves.

(iii)

Biotic and abiotic factors promoting deposition of the calcretes were identified from application of geochemical assays of XRD, optical petrography, elemental chemistry, and carbon and oxygen isotopes.

(iv)

Evaporation of vadose groundwater, CO₂ derived from microbial and root respiration lowering the fluid pH and causing limestone dissolution, release of Ca in solution, and calcite supersaturation are the primary factors controlling the pedogenic calcrete formation.

(v)

Hydrotalcite minerals [Mg₆Al₂(CO₃)(OH)₁₆(H₂O)₄] are exceptionally rare in nature, and therefore, the occurrence of a hydrotalcite-rich calcrete is unique.

(vi)

The plausible mechanism of hydrotalcite deposition is interaction between dolomite and seawater, a process analogous to hydrotalcite synthesis in the laboratory.

(vii)

Calcretes are important archives of the ambient terrestrial environments and ecosystems. Systematic sampling and laboratory studies of the calcretes in Niue Island offer valuable records of past rainfall variability and soil development changes in a understudied region of the Southwest Pacific impacted by droughts attributed to SPCZ and ENSO events.