Alvin Dive 3349 - Summary

(On-axis, Animal Farm at 18°36.5'S to 18°37.4'S)
Observers: Cindy Van Dover ( transcript) and Milène Cormier (Transcript)
Pilot: Steve Faluotico

Summary: The objective for the first part of the dive was to locate and sample a mussel bed at Animal farm for biodiversity comparisons. We returned to the coordinates of the mussel bed sampled on Dive 3343 (Animal Farm Site 1; x=3525, y=8433), measured temperature (ambient=1.74°C; within mussels=1.84°C), and collected one large (2-liter) box of mussels using a scoop. Subsequent to this sampling, we undertook a short survey of the area in search of a mussel bed with more vigorous water flow and healthier mussels. Not finding this, we returned to within 20m of our site (Animal farm Site 2; x=3530; y=8420) where there was a larger (10m vs 4m max dimension) mussel bed. Maximum temperature measured in this mussel bed was 2.6°C. We tested one mussel pot sampler but it failed due both to the single layer of mussels making up the bed and to a snag in the cinch line. Five scoop samples (each of ~2 liter volume) were collected from this site as well as an incidental rock sample which was given a sample number post-dive.
The objective for the second part of the dive was to survey the system of right-stepping en echelon fissures visible in the DSL-120 side scan record along the ridge axis. We proceeded southward from the Animal Farm site and zig-zagged across the fissures to assess their nature, width, and dimensions. All the lava flows encountered were very glassy, with only a light dusting of sediments; sessile macrofauna was present almost continuously, and anemones were a common sight. We estimate a relative age of the lavas of 1.0 on the Macdonald age scale, similar to previous age estimates for the Animal Farm lava flow. Each crossing of the fissures revealed similar features. The depths of the fissures estimated from Alvin's pressure sensor and down-looking sonar vary between 5 and 9 m. Their widths are estimated to be 40-60m (although all crossings were oblique to their strike, DSL 120 records suggest a strike of N010° near 18°36'S). Untectonized lobate flows occurred a short distance from the fissures, sometimes grading into sheet flow and jumbled sheet flow near their rims. The walls of the fissures never exposed fresh break (= these fissures are eruptive fissures). Sheet flows draping down the walls of the fissures like waterfalls were observed on several occasions. The upper crust of these cascading sheet flows is fragmented into imbricated pieces in the upper reaches of the wall (= the fragments are still plastered on the cascading sheet flow, frozen-in-action). Down in the face of the walls, fragmented sheet flows were common, and behind the draped flow, "bathtub-ring-like" layering a few cm thick. Lava pillars have been observed only near 18°36.8'-36.9'S. Most of the time, the floor of the fissure was lined by lineated sheet flow (with lineations striking sub-parallel to the trend of the fissure), and jumbled, fragmented sheet flow. These observations suggest a high eruption rate which infilled the fissure and flooded the flanks with lobate flows, followed by a drainback so rapid that the lavas just erupted on the rims cascaded back down to the bottom of the fissure.

Samples collected

# 	      Time	 Latitude(S) Longitude(W)Depth (m)	Site	Description
3349-1	10:37	18°36.439'	113°24.025'	2677	Animal Farm	mussel bed sampled with "mussel pot" (turned out to be empty)
3349-2	10:48	18°36.433'	113°23.993'	2677	Animal Farm	~2 liters of mussels; fragments of jumbled sheet flow
3349-3	11:51	18°36.433'	113°23.993'	2677	Animal Farm	~2 liters of mussels
3349-4	12:06	18°36.433'	113°23.993'	2677	Animal Farm	2 liters of mussels
3349-5	12:20	18°36.433'	113°23.993'	2677	Animal Farm	2 liters of mussels
3349-6	12:59	18°36.431'	113°23.997'	2677	Animal Farm	2 liters of mussels
3349-7	13:02	18°36.431'	113°23.993'	2676	Animal Farm	2liters of mussels
3349-8	13:37	18°36.431'	113°23.993'	2676	Animal Farm	2 liters of mussels 
3349-9	13:54	18°36.664'	113°23.958'	2672	east of ASC	large slab of lobate flow
3349-10	14:39	18°37.081'	113°24.152'	2679	bottom of ASC	3 pieces of sheet flow
3349-11	15:03	18°37.165'	113°24.167'	2679	bottom of ASC	small piece of sheet flow


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