Physical Description
Sample location 001 was collected from a diamicton facies (Bosch and White 2004) at the end of a passage known as Dave’s Gallery (Fig. 1). One core sample was collected from the top of sediment bank with a recovery of 26 cm and three grab samples were collected from the face of the sediment bank between the bottom of the core toward the cave floor (Fig. 2a). The sediment bank consisted of a finer grained sandy cap over a thicker, gravel sized deposit (Fig. 2a). Based on field observations, the coarser sediments were poorly sorted, angular to subangular, and unsaturated. The core sample was split open in the laboratory and appeared to be consistent in grain size and color (brownish yellow, 10 YR 6/6 on the Munsell color chart. Although no visible change in grain size or color was observed, the core was subsampled in two sections of roughly equal length for analyses (Fig. 2a). Sample location 002 was collected from the dry stream bed in a passage known as Sand Canyon. Although the stream was dry at the time of sampling, it is known to flow intermittently. Two consecutive cores with total recovery of 43 cm and one grab sample from the top of the cored area were collected. The core samples were subsampled in four sections based on visible change in grain size. The color of the core samples was dark yellowish brown (10 YR 4/5 Munsell color chart), and no distinct stratification or layering was observed. Sample location 003 was collected from the same bank deposit as 002 but slightly downstream and higher (relative to the stream bed) in the deposit stratigraphy than location 002. One core sample with 22 cm of recovery was collected and one grab sample from the bottom of the core was collected. Overall, the core sample was observed to be yellowish brown in color (10 YR 5/6 on the Munsell color chart). Sample location 004 was collected from an approximately 225 cm tall bank around the bend from Sand Canyon. Two consecutive core samples were collected from the top of the bank with total recovery of 45 cm. Five grab samples were collected from the face of the bank moving down to represent the entire bank from top to bottom, save for the last ~ 50 cm which consisted of large gravels and small boulders. The sediments appeared to be well sorted but varied in grain size from sand to large boulders (Fig. 2b). The core samples were subsampled in 10 total sections. The top 20 cm of the core had distinct layers of sand and silt sized particles that were light yellowish brown (10 YR 6/4) and dark yellowish brown (10YR 4/4), respectively. The bottom 17 cm of the core was mostly sandy with a smaller, clay-like layer between 14 – 16 cm (Table 1). The sandier bands were much thicker than the smaller particle size bands and were light yellowish brown (10 YR 6/4) while the smaller particle band was dark yellowish brown (10 YR 4/4). Sample locations 002 through 004 represent the channel facies (Bosch and White, 2004). Sample location 005 was collected from a sediment bank consisting of mostly sand sized particles capped by a smaller silt or clay-like textured sediment. One grab sample of the sand and one of the clay-like sediment were collected. This location has not been previously categorized according to the facies classification system but may represent channel or slackwater facies. Finally, sample location 006 was collected from a sediment bank further back in the cave and close to an active stream. One core sample of 23 cm recovery was collected and subsampled in four sections. While above water at the time of sampling, this core contained much more moisture when split open than other sediment cores collected in during this sampling event. This location was also not previously categorized in the facies classification scheme but may represent slackwater facies. The total list of samples and sample naming schema is listed in Table 1.
Particle Size Analysis: Active Fraction, < 2mm
All samples were sieved 2 mm to obtain the active size fraction and these materials were analyzed for particle size. All samples at locations 001, 002, 003, 004, and 005 were classified as some type of sand (Table 1), ranging from fine to coarse, except for four samples at locations 001 and 002 that were classified as fine to coarse silts (Table 1). All the samples from location 006 were classified as course silts (Table 1). The volume percent for all the samples ranged from 16.2% – 91.4% sand, 6.1% – 83.8% silt, and 0% – 12.7% clay. The highest sand content was at location 003 and the lowest was at location 006 (Fig. 3).
The TC concentrations ranged from 0.08 – 0.87%: the lowest concentration was at location 006 and the highest at 003. TOC also ranged from 0.08 – 0.87 % because some samples had zero measurable TIC concentrations. Location 002, in the active cave stream, had the highest average TOC while location 004 (the location with the largest grain size) had the lowest. A linear regression between TC and TOC data including all samples had slope = 0.91 (R2 = 0.92), indicating that most of the carbon in all the samples was TOC. Location 001 had the largest range of TOC with slight positive skew while location 004 had a smaller range of TOC with close to normal distribution. Location 003 had the highest median TOC and location 006 had lowest median TOC (Fig. 4a).
Microbial uptake of nitrogen is a driver of organic carbon decomposition in surface leaf and root litter (Ravn, 2020); however, in caves total nitrogen is often low (and as a result the C:N ratio is low), thus slowing down the rate of organic carbon decomposition. (Ravn, 2020). Reporting TOC and N concentrations can provide information on how microbial activity may be supported in the caves and how this may contribute to carbon processing in caves. TOC:N ratios were compared to the average C:N ratios of amino acids, 3.15, (Jover et al. 2014) and C:N range of humic and fulvic acids, 6.23 - 147, (Rice and MacCarthy 1991), respectively. The C:N ratio of amino acids generally represents a microbially-based source of organic carbon (Jover et al. 2014) whereas humic and/or fulvic acids represent the heterogenous, molecular organic components of soil organic matter (Rice and MacCarthy 1991). The TOC:N molar ratio range across the six locations was 3 – 15 with the highest and lowest concentrations represented at locations 006 and 003, respectively. Location 001 and location 003 have the largest range of TOC:N but location 003 has the highest median and average of TOC:N. Location 006 has the lowest median TOC:N (Fig. 4b). Most of the TOC:N data fall above the average amino acid ratio and slightly below or well within the humic and fulvic acid range (Fig. 5). This could possibly indicate that the TOC in these samples is largely a result of soil organic matter that has been washed into the cave. Interestingly, three out of the four samples collected from the core at location 006 had a TOC:N ratio at or below 3.5 and this location also had the lowest average TC and second lowest average TOC. This could possibly indicate that the further away from source input a sample is, the less likely terrestrial OC is to be washed that deep and that microbial signatures of TOC dominate any TOC that is present even if TOC concentrations are comparatively lower.
Some of the sample locations had a positive linear relationship between the percentage of silt-sized particles and TOC but this was not true across the entire sample set. A positive linear relationship was observed in each of the diamicton facies (location 001) and channel facies (location 004) between the silt-sized fraction and TOC but not between the clay-sized fraction and TOC. For the channel facies, when the core and grab samples were compared individually, the core samples had a linear relationship in silt-sized particles and TOC, but the grab samples did not. At location 006 where silt was the dominant sized fraction, a negative linear relationship was observed between the silt-sized fraction and TOC, but a positive linear relationship was observed between the clay-sized fraction and TOC. It should be noted that small sample sizes and data clusters around high silt-size percentages and low clay-size percentages could be skewing these relationships. The data presented here show that large ranges organic carbon content can occur in clastic cave sediments across grain sizes and lithofacies.
Comparison of channel and diamicton facies, > 2 mm fraction
The samples collected at locations 001 and 004 were selected to compare the diamicton (poorly sorted, chaotic dump of cobble to silt size particles resultant from large debris flows into the cave) and channel facies (moderately sized, moderately sorted interbedded silts and sands that often show stratification and are deposited by active cave streams). These locations were chosen based on previous mapping and descriptions (Bosch and White 2004) and each represent a potential different mechanism of clastic sediment deposition and processing in caves.
For the active fraction of the diamicton facies (location 001), the samples mostly consisted of sand-sized grains although the three grab samples had slightly more silt than the core samples (Fig. 6a). In the diamicton facies, TOC ranged from 0.08 – 0.37 % (Fig. 6b) and TOC:N ranged from 4 – 7.4 (Fig. 6b). There was an increase in TOC and TOC:N with depth from the top of the bank with the exception of location 1E (the bottom of the bank) which had a marked decrease in TOC and TOC:N (Fig. 6c, d). This could be due to the overall large grain size dominating the sample, based on field observations. Most of the TOC:N ratios fell between the average amino acid range but below the range of humic and fulvic acids (except for 1D). Given the inferred source of a diamicton facies, these samples are likely dominated by terrestrial TOC. There is a decrease in grain size with depth of sample (Fig. 6c) with sand making up 86% of the active fraction at the top of the bank and decreasing to only 45% at the bottom. The observed overall grain size at the bank (sand to boulder) was observed to increase from the top of the bank to the bottom of the bank. However, since this facies represents a large, chaotic injection of sediment, it is not advised to interpret this as a depositional feature with regard to depth in the same way one would consider it in a surface sedimentological setting.
Welch’s t-test was performed to compare the means values of the sand, silt, clay, TOC, and TOC:N between the grab (n = 2) and core (n = 3) samples at location 001. The result showed a significant difference (a = 0.05) in the mean for the sand and silt size fractions (p 0.05) where the mean sand-size percentage was greater in the core samples and the mean silt-size percentage was greater in the grab samples. The remaining variables have no significant difference in means. However, because each sample size is small, these results should be interpreted with caution since Type I or Type II errors can be common in low sample size populations.
In the channel facies at location 004, the bank is capped by a sandy deposit approximately 47 cm thick and then increases in grain size down the bank from gravel to boulder. The samples are mostly sand sized with some core samples being slightly siltier than the grab samples (Fig. 7a). The core samples have alternating high percentages of sand and silt with depth in the first 20 cm which is consistent with described interbedded sands and silts that are descriptive of channel facies. The grab samples had a more consistent grain size with depth which is likely due to an overall larger grain size (cobble and boulder) dominating the deposit as observed during sample collection. The TOC ranged from 0.1 – 0.26 % and TOC:N ranged from 4.7 – 8.7 (Fig. 7b) with six samples in the humic and fulvic acid range and nine samples between the humic and fulvic acid range and the amino acid average (Fig. 7b). The TOC source in these samples is likely terrestrial. The core samples show an alternating pattern of increasing and decreasing TOC concentrations and TOC:N ratios with depth (Fig. 7c, d) which is indicative of the interbedded sands and silts characteristic of channel facies. Results from Welch’s t-test between the core and grab samples at location 004 also showed a significant difference (a = 0.05) in the means of the sand and silt size fractions (p 0.05) where the mean sand percentage was greater in the grab samples and the mean silt percentage was greater in the core samples. Differences in the core and grab samples could be due to the overall differences in grain size or exposure of the grab samples to the ambient cave environment.
A significant difference (a =0.05) was observed between the core samples at location 001 and 004 for the means of sand, silt, and TOC; the mean sand percentage was greater in the diamicton facies (001), but the mean TOC and silt percentage was greater in the channel facies (004). Since organic carbon is often associated with smaller size fractions, it is reasonable that a siltier sediment (004) will have more TOC than a less silty sediment, which is supported by these statistics. Grab samples between the diamicton and channel facies had a statistical difference (a = 0.05) only in the sand and silt size fractions where the mean sand percentage was greater at in the channel facies and mean silt was greater in the diamicton facies. It is possible error is present in this analysis due to the large difference in sample size (location 004 had three times more samples collected than location 001) and the overall low number of samples available at 001. The observed differences in the data suggest there could be a difference in the active fraction particle size and chemistry of these two sediment lithofacies. The differences in core and grab samples could be due to the sampling method, where coring is more likely to capture smaller particles than grab sampling. However, the core samples in the diamicton facies were sandier (coarser) than the grab samples.