Supplementary information

Stable isotope methodology
Hand-specimens were drilled using a 0.3mm drill bit to extract 1mg sub-samples of carbonate powder for stable isotope analysis at SUERC, VU University Amsterdam, and Cardiff University. Carbon and oxygen isotope data are presented here as δ values in permil relative to VPDB. Three selected thin-sections (ATB Moor Cliffs 5; ATB 210810-7; ATB 220810-5) were micro-sampled using a computer-controlled Merchantek micromill at VU University Amsterdam, but only one of these thin-sections yielded sufficient carbonate powder for analysis. 

For stable isotope analyses at SUERC, carbonate powders were dissolved in 103% phosphoric acid at 70ºC overnight, and CO2 produced was analysed on either a VG PRISM III or an AP 2003 mass spectrometer. Repeat analyses of NBS-18 and internal calcite standards are generally better than +/- 0.2 ‰ for carbon and 0.3‰ for oxygen. For one mixed calcite-dolomite sample, ~30 mg of powdered sample were first reacted with 103% phosphoric acid at 25 ºC for 1.5 hours. The resulting CO2 (presumed to be from the calcite) was then extracted on a vacuum line and measured on a VG SIRA 10 mass spectrometer. The remaining sample was then heated at 100 ºC overnight to dissolve the less soluble (dolomite) component, from which CO2 which was similarly extracted and measured, following procedures of Al Aasm et el. (1990) and using appropriate fractionation factors for calcite at 25 ºC (103lnα =10.25; SUERC unpublished data) and dolomite at 100 ºC (Rosenbaum and Sheppard, 1986). 

For stable isotope analyses at VU University Amsterdam, carbonate samples of 50 μg weight were dissolved in individual tubes of phosphoric acid overnight at 45 ºC and analysed using a ThermoFinnigan Delta Plus with GasBench II. Repeat analyses of an internal calcite standard calibrated to NBS-19 are generally better than ±0.1‰ for carbon and ± 0.15‰ for oxygen. 

Calcrete samples collected from Tredomen Quarry were analysed at Cardiff University using a Finnigan MAT252 mass spectrometer and calibration to NBS-19. Repeat analyses of  NBS-19 are generally better than ±0.1‰ for both carbon and oxygen.

Samples of organic matter (extracted with a tungsten needle from coalified compressions of rhyniophytoid axes and of both charcoalified and coalified Prototaxites fragments) were obtained from a plant assemblage from Tredomen Quarry (Morris et al., 2011) and were treated with a small amount of HF, followed by boiling HCL (40%). Their δ13C values were measured at Newcastle University, UK and Iso-Analytical Ltd. Here, the samples were acid washed using 10% HCl, followed by 3 washes in distilled water. They were oven dried and weighed into tin samples for flash combustion in a Europa Scientific elemental analyser at ~1700 °C, coupled to a Europa 20-20 isotope ratio mass spectrometer. Repeat analyses of reference standards (IA-R001, IAEA CH-6 and IA-R005) gave precision of better than 0.2‰.

Supplementary information palaeoatmospheric pCO2 calculation methods:

Calculation of the carbon isotopic composition of palaeoatmospheric CO2 (δ13Cair) follows Schaller et al. (2011).  Using δ13Corg of -25‰, and assuming consistent fractionation by photosynthesis, a value of -5.75‰ is obtained:

δ13Cair = (δ13Corg + 18.67)/1.10

References:
AL-AASM, I.S., TAYLOR, B.E., and SOUTH, B., 1990, Stable isotope analysis of multiple carbonate samples using selective acid extraction. Chemical Geology, 80, 119-125.
Morris, J.L., Richardson, J.B. & Edwards, D. 2011. Lower Devonian plant and spore assemblages from Lower Old Red Sandstone strata of Tredomen Quarry, South Wales. Review of Palaeobotany and Palynology, 165, 183-208.
ROSENBAUM, J. M., SHEPPARD, S.M.F., 1986. An isotopic study of siderites, dolomites and ankerites at high temperatures. Geochimica et Cosmochimica Acta, 50, 1147-1159.

Thin-section petrography of Moor Cliffs Formation calcretes
Calcrete specimen (ATB MC5) from Manorbier (Table S1) is an elongate, 15mm long and 3mm wide nodule. In thin-section, the matrix around the nodule comprises clays and sub-angular siliciclastic fine sand grains (Figs. 3a and 3b). The carbonate fractions comprise patches of micrite, which appear buff-yellow in polarized light, with former cavities infilled by clear, sparry calcite cement. The sparry calcite is never found without a micrite coating, and always occurs as cement within clast-free irregularly-shaped voids, that could represent burrows or root moulds. 

A 10 cm diameter nodule taken from the top of the Chapel Point Limestone at Llansteffan (ATB 190810-01; Table S1) exhibits a crystalline mosaic of microspar that includes some mm-sized grains coated with 100 micron-thick clotted fabrics of potential microbial origin (Fig. 3d). There is evidence of a weak primary layering. Conjugate fractures in the most brittle grains are filled with spar, and the same generation of spar is found as circum-granular coatings on some peloids. 

Thin-section petrography and sampling of Freshwater West Formation calcretes
A thin-section of part of composite pedogenic nodule ATB 220810-09 from the Rat Island Mudstone Member at Freshwater West shows a uniform crystalline appearance (Fig. 5c). The matrix is cut by stylolites and millimeter-scale ‘veinlets’ of sparry calcite. In places the veinlets seem to follow the stylolite (perhaps implying the latter formed a conduit for syn- or post-burial fluid flow), but the veinlets are also terminated by the stylolites. It is therefore hard to establish the timing of the veinlets in relation to the stylolites.

A thin-section of a nodule ATB 190810-7 collected from a conglomerate (interpreted as a crevasse splay deposit) in the Freshwater West Formation at Llansteffan reveals calcitic microspar with a spherulitic appearance (Fig. 5e). Individual spherulites are up to 300 microns in diameter. However the sub-crystals of these spherulites do not exhibit sweeping extinction, suggesting that some fabric-preserving recrystallisation has taken place. Around the outside of the spherulite-containing clast, separating it from the siliciclastic palaeosol matrix, is a laminar calcite crust of c. 100 microns thickness and c. 5mm length (Fig. 5f). Three couplets of light and dark laminae (as seen in transmitted light) can be observed. 

Thin-section petrography and sampling of Tredomen Quarry core calcretes
The five selected nodules from Tredomen Quarry cores (Table S1) are petrographically similar, being centimetre-scale, sub-spherical to cylindrical in shape, and set in homogenous mudstone matrices (e.g. nodule 1, Fig. 6). The nodules have sharp to slightly diffuse boundaries and are commonly surrounded by circumgranular cracks. Where ca. 1mm thick spar-filled cracks cut through the nodule micrite they do not extend into the mudstone matrices. Several of the nodules are in fact composite, comprised of several smaller coalesced micritic peloids cemented together by calcite spar (crystals being up to 1mm in diameter). Within the nodules, the largest crystals are mostly less than 50 µm in diameter. In one of the five examined cases (Nodule 9; Table S1), the calcite crystals show a slight increase in diameter from the centre of the nodule outwards, to a maximum of 0.05mm diameter. Internal cracks are filled with spar up to 0.5mm in diameter. Small patches of segregated iron line the spar-filled cracks. Floating sand grains are rare but are encountered, commonly exhibiting corroded margins. 

Thin-section petrography of Ridgeway Conglomerate Formation calcretes
A thin-section of nodule ATB 220810-11 (Table S1) reveals millimetre-sized dark micritic peloids coated in c. 100 micron-thick layers of calcitic microspar (Fig. 7c). These microspar crystals exhibit clear terminations. 

Bulk micritic samples of the nodules were taken for stable isotope analyses.

Sample	Location	Formation	Member or position	Calcrete type	Description of fabric sampled	δ13C (VPDB)	δ18O 
(VPDB)	Thin-section	
ATB MC 1	Manorbier	Moor Cliffs	3 m above Rooks Cave Tuff	10 to 15 mm diameter nodules	Pink nodule micrite	-10.11	-12.8		
ATB MC 2	Manorbier	Moor Cliffs	1 m above Rooks Cave Tuff	5mm diameter round nodules	Grey-green nodule micrite	-10.5	-13.0		
ATB MC 3	Manorbier	Moor Cliffs	1m below Rooks Cave Tuff	0.5 cm wide, 1.5 cm long flattened tubes, possible burrow fills?	Pink micrite cut by clear spar	-9.7	-12.7		
ATB MC 4	Manorbier	Moor Cliffs	5m below Rooks cave Tuff	15 mm long, 2 to 3 mm wide nodules (burrow fills?)	White nodule micrite	-10.3	-12.7		
ATB MC 5	Manorbier	Moor Cliffs	5m below Townsend Tuff	15 mm long, 3 mm wide elongate nodules 	Grey-green nodule micrite	-8.0	-12.0		
ATB 210810-01	Manorbier	Freshwater West	Conigar Pit Sandstone	Large pedogenic nodules	Pink nodule micrite	-11.5	-11.6		
ATB 210810-02	Manorbier	Freshwater West	Conigar Pit Sandstone	Elongate nodules, 3 cm long, 1.5 cm wide	Pink nodule micrite cut by white sparry crystallaria	-11.8	-12.2		
ATB 210810-03	Manorbier	Freshwater West	Conigar Pit Sandstone	Small < 1 cm diameter calcrete clasts	Centre of a white micritic nodule, 0.4 cm wide, 0.7 cm long	-11.7	-12.2		
ATB 210810-04	Manorbier	Freshwater West	Conigar Pit Sandstone	Small elongate nodules, 2 cm long, 0.4 cm wide	Grey-green nodule micrite	-10.0	-11.2		
ATB 210810-05	Manorbier	Freshwater West	Conigar Pit Sandstone	Small elongate nodules	White weathering micrite	-11.1	-12.4		
ATB 210810-06	Manorbier	Freshwater West	Conigar Pit Sandstone	Large composite pedogenic nodules, with each sub-nodule c. 1 cm diameter, cemented together by white spar	Centre of a pink nodule micrite cut by clear sparry crystallaria 	-11.1	-11.7		
ATB 210810-07	Manorbier	Freshwater West	Conigar Pit Sandstone	Large pedogenic nodules	Centre of a composite nodule, with several different micritic nodules (each up to 3.5 cm diameter) cemented together. Sparry calcite seen within some of the micritic nodules.	-10.1	-9.2		
ATB 210810-07 (calcite fraction, dissolved at 25oC)	Manorbier	Freshwater West	Conigar Pit Sandstone	Calcite from nodule		-11.2	-9.9		
ATB 210810-07
(dolomite fraction, dissolved at 100oC)	Manorbier	Freshwater West	Conigar Pit Sandstone	Dolomite (burial) cement from nodule		-12.1	-10.8		
ATB 210810-08	Manorbier	Freshwater West	Conigar Pit Sandstone	Large pedogenic nodules	Centre of a 3 cm long, 1.5 cm wide green-white micritic nodule	-11.1	-9.4		
ATB 210810-09	Manorbier	Freshwater West	Conigar Pit Sandstone	Large pedogenic nodules	Centre of a white weathering 1 cm diameter nodule, internally comprised of red micrite and grey-green to red micrite and microspar	-11.3	-13.1		
ATB 210810-10	Manorbier	Freshwater West	Conigar Pit Sandstone	Small 1 cm long, 0.5 cm wide nodules	Centre of a pink micritic nodule cut by white sparry crystallaria	-11.6	-13.6		
ATB 210810-11	Manorbier	Freshwater West	Conigar Pit Sandstone	Large calcrete conglomerate clasts	Pink micritic nodule cut by white sparry crystallaria	-10.7	-12.3		
ATB 210810-12	Manorbier	Freshwater West	Conigar Pit Sandstone	Pedogenic nodules, composite, up to 3.5 cm diameter	Pink to white nodule micrite	-10.1	-10.9		
ATB 190810-01	Llansteffan	Moor Cliffs	Chapel Point Limestone	10 cm diameter pedogenic nodule	Grey-green coloured nodule micrite	-9.2	-9.1		
ATB 190810-02	Llansteffan	Moor Cliffs	Chapel Point Limestone	Vertically oriented nodule rods	Grey-green coloured nodule micrite, part of a c. 5 cm diameter nodule	-9.6	-11.1		
ATB 190810-07	Llansteffan	Freshwater West		Pedogenic nodules, 2 to 3 cm diameter	Pink nodule micrite, with thin (<1 mm diameter) white sparry calcite crystallaria	-11.2	-12.2		
ATB 190810-08	Llansteffan	Freshwater West		Vertical nodule rods	Pink nodule micrite	-10.9	-11.5		
ATB 190810-09	Llansteffan	Freshwater West		Pedogenic nodules, c. 3 cm diameter	Grey nodule micrite to microspar	-10.7	-12.5		
ATB 190810-10	Llansteffan	Freshwater West		Conglomerate nodules	Centre of a 1 cm diameter micritic calcrete pebble	-10.1	-11.8		
ATB 190810-11	Llansteffan	Freshwater West		Vertically oriented nodule rods, 3-4 mm wide and up to 3 cm long	Pink nodule micrite, but may also have included clear spar in centre of nodule	-9.8	-12.0		
ATB 220810-02	Freshwater West	Freshwater West	Conigar Pit Sandstone	Small calcrete clasts	Pink nodule micrite	-10.8	-13.4		
ATB 220810-03	Freshwater West	Freshwater West	Conigar Pit Sandstone	Crystallaria / elongate nodules	Weathering-resistant pink-white irregular-shaped micritic nodule, with sparry calcite (crystallaria) centre	-9.1	-5.8		
ATB 220810-05	Freshwater West	Freshwater West	Conigar Pit Sandstone	Large pedogenic nodules	Pink nodule micrite cut by white sparry crystallaria	-10.4	-12.3		
ATB 220810-5-1	Freshwater West	Freshwater West	Conigar Pit Sandstone	Large pedogenic nodules	Middle of crystallaria	-10.0	-8.1	*	
ATB 220810-5-2 	Freshwater West	Freshwater West	Conigar Pit Sandstone	Large pedogenic nodules	Edge of crystallaria 	-10.1	-13.6	* 	
ATB 220810-5-3 	Freshwater West	Freshwater West	Conigar Pit Sandstone	Large pedogenic nodules	Nodule micrite	-10.1	-13.8	*	
ATB 220810-06	Freshwater West	Freshwater West	Conigar Pit Sandstone	Large 3 cm diameter pedogenic nodules	Pink nodule micrite	-10.1	-8.3		
ATB 220810-07	Freshwater West	Freshwater West	Conigar Pit Sandstone	Crystallaria	Sparry calcite crystallaria	-11.1	-12.9		
ATB 220810-08	Freshwater West	Freshwater West	Conigar Pit Sandstone	Large nodule (5 cm diameter), possibly groundwater-influenced	Grey-green coloured nodule micrite	-10.0	-9.0		
ATB 220810-09	Freshwater West	Freshwater West	Rat Island Mudstone	Pedogenic nodules, composite, with 0.5mm cores and up to 5 cm diameter in total	White weathering micritic nodule	-10.1	-12.6		
ATB 220810-10	Freshwater West	Freshwater West	Rat Island Mudstone	Pedogenic nodules	Sparry calcite (crystallaria) around outside of a 0.8 cm diameter red micritic nodule	-9.9	-14.0		
ATB 220810-11	Freshwater West	Ridgeway Conglomerate		Pedogenic nodules with crystallaria	Grey crystalline nodule, at least 1 cm x 2 .5 cm diameter, part of a larger nodule that is cross-cut by crystallaria	-9.0	-13.8		
ATB 220810-15	Freshwater West	Ridgeway Conglomerate		Pedogenic nodules with crystallaria	Sparry calcite crystallaria	-8.8	-12.9		
ATB 220810-12	Freshwater West	Ridgeway Conglomerate		Pedogenic nodules	Elongate 3 mm-wide and 1 cm long nodule of green to white coloured calcite microspar, next to a larger 3 cm diameter red micritic nodule	-8.8	-13.7		
ATB 220810-13	Freshwater West	Ridgeway Conglomerate		Very large nodule	Outer zone of nodule, crystalline and sparry	-6.7	-12.9		
ATB 220810-14	Freshwater West	Ridgeway Conglomerate		Pedogenic nodules, 2 to 3 cm diameter	Red-coloured nodule micrite and veinlets of white sparry crystallaria	-7.8	-14.1		
Tredomen Nodule 1	Tredomen Quarry 	Freshwater West	†2.4m	Pedogenic nodule	Nodule micrite	-8.1	-11.4		
Tredomen Nodule 4	Tredomen Quarry 	Freshwater West	†18.95m	Pedogenic nodule	Nodule micrite	-11.8	-8.4		
Tredomen Nodule 8	Tredomen Quarry 	Freshwater West	†55.1m	Pedogenic nodule	Nodule micrite	-10.9	-10.8		
Tredomen Nodule 9	Tredomen Quarry 	Moor Cliffs	†63.4m. Chapel Point Limestone	Pedogenic nodule	Nodule micrite	-9.8	-9.6		
Tredomen Nodule 12	Tredomen Quarry 	Moor Cliffs	†73.7m	Pedogenic nodule	Nodule micrite	-9.3	-9.5		
ATB-RH Crystallaria	Freshwater West	Freshwater West		Sparry calcite crystallaria	White coloured sparry calcite crystallaria (c. 2mm wide, 3cm long)	-8.6	-12.0		

Table S1: Carbon and oxygen isotopes of Siluro-Devonian calcretes from South Wales, UK.  = thin section prepared. * = sampled from thin section. † = sample depth from Core SO13SW/3 (British Geological Survey).