Table 1.
Vibrational band wavenumbers, assignment and potential molecular origin in biological material, as compiled by Movasaghi et al. [39] and Preston et al. [40].
Fig 1.
Some of the remarkable tubular alteration textures in the Louisville samples in volcanic glass under plane polarized light.
A (U1376A-16R-6W-65/71): Long, thin, and branched tubular alteration textures preserved in palagonite and indicated by white arrows. B (U1376A-16R-6W-65/71): A single long, wide, and branched tubular alteration texture preserved in glass. C (U1376A-16R-6W-65/71): Several very large tubular alteration textures preserved in palagonite and indicated by white arrows. D (U1372A-19R-1W-79/81): Remnants of tubules preserved (or formed) in palagonite (white arrows), red arrows show them protruding into fresh glass. E (U1372A-29R-3W-77/80): Long and wide tubule, which shows significant variation in width at the terminus along with simple branching F (U1376A-16R-6W-65/71): Significantly large textures preserved in palagonite, similar to those shown in D. The length of the scale bars is 50 μm in A, 20 μm in B, 200 μm in C, 100 μm in D, 20 μm in E, and 100 μm in F.
Fig 2.
Types of tubular alteration textures found in the North Pond and Louisville samples.
In Louisville basalt glass, short and thin tubes make up almost 50% of the samples, but longer and thicker tubes are prevalent as well, compared to the North Pond samples. Furthermore, Louisville samples are more diverse in terms of variations in widths, branching, and tunnel contents, compared to the North Pond samples. For North Pond basaltic glass, thin tubes whether long or short, make the majority of the samples, and thick tubes are almost completely absent. They do not show any branching, are exclusively empty, and do not show any variations in width.
Fig 3.
North Pond sample U1383C-29R-1W-6/9: SEM image (backscattered electrons (BSE)) and associated EDS elemental maps for C, Ca, Si, Fe, and Mg collected at 15 kV accelerating voltage.
Carbon coming from the organic matter, appearing as dark spots of less than ten micrometers on the BSE image, is located closely to the Fe-enriched palagonite/Ca-enriched glass interface. They are particularly concentrated in the glass hollow from which palagonite was removed.
Fig 4.
Typical FTIR spectrum of the organic matter found in the North Pond sample U1383C-29R-1W-6/9.
The presence of organics is suggested by the following absorption bands: 1: CH3 asymmetrical stretch, 2: CH2 asymmetrical stretch, 3: CH2 symmetrical stretch, 4: C = O stretch, 5: Amide I band, 6: Amide II band, 7: CH3 and CH2 scissoring and asymmetric bends, 8: Amide III band (Table 1). The broad main around 1250 cm-1 absorption band corresponds to the Si-O bond. A.U. stands for arbitrary unit.
Fig 5.
Typical FTIR spectrum of the organic matter found in the Louisville sample U1376A-16R-6W-65/71.
The absorption bands indicate the presence of organics. 1: is the CH3 asymmetrical stretch, 2: is the CH2 asymmetrical stretch, and 3: the CH2 symmetrical stretch. The broad main around 1250 cm-1 absorption band corresponds to the Si-O bond. A.U. stands for arbitrary unit.
Fig 6.
(A) “negative” split of a basalt glass from Louisville sample U1376A-16R-6W-65/71 (resin-free thin section), as shown under plane polarized light; (B) associated FTIR map highlighting the spatial distribution of the CH3 vibrational band. The FTIR map location is shown by the black rectangle in (A) and indicates highest abundance of organic matter in the palagonite-filled vesicle, from which tubular alteration textures protrude into the surrounding volcanic glass. The scale bars are 200 μm (left) and 100 μm (right).
Fig 7.
SEM images in BSE mode of a vesicle in a basalt glass (light grey) filled with palagonite (dark grey) (Louisville sample U1376A-16R-6W-65/71).
Several etch pits are visible on the vesicle surface on the right; the image on the left shows a close up view of an etch pit, which protrudes into the volcanic glass as the onset of a tubular alteration texture.
Fig 8.
R3/2 values as deducted from FTIR analysis of the organic matter found in the North Pond and Louisville samples.
The values mostly plot above the values of whole bacterial and eukaryotic cells in both cases [43;44]. Note that, individual cell constituents like membranes and lipids have lower R3/2 values than whole cells, whereas proteins plot higher.
Fig 9.
top: SEM image of Louisville sample U1376A-15R-3W-37/41 in BSE mode showing organic matter microaggregates (1 & 2) within a palagonite rim (3 & 4) on top of fresh glass with several etch pits and below an SiO2 filled fracture (5). bottom: sketch as side view (bottom left) and top view (bottom right), illustrating the spatial distribution of the tubular biosignatures and the organic matter remnants. The tubes originate from the fracture/fresh glass interface and propagate into the glass, are then overgrown by palagonite and sealed by precipitating SiO2. The organic matter was only found within palagonites, suggesting that these putative remnants of microbial life were not associated with formation of tubular alteration textures.