Month: October 2016

Piston Core 36

Jennifer Gonzales

In previous posts we have discussed deep-sea sediments that are rich in skeletal debris. Now we will focus on a site that is abundant in siliciclastic material. Piston core 36 (PC36) is a site located adjacent to the Western Sepik River at a water depth of 903 m. The sample we will be focusing on was collected at a subsurface depth of 139-140 cm. Its close proximity to the delta may the source of such terrigenous material.

 

The amount of sediment a river can transport changes over time, therefore, sediments found at varying depths can reveal changes in precipitation. Change in precipitation is a result of climate change, and as climate changes so do the environments that are in the region! A comparison of siliciclastic material will be addressed in an upcoming post where we will look at another sample from PC36, but at a subsurface depth of 218-219 cm. Siliciclastic rocks are non-carbonate sedimentary rocks that are almost exclusively silica bearing. They are terrigenous material, derived from erosion or weathering of land-based rocks, transported by wind or water. Common siliciclastic minerals are quartz, feldspars, micas, and heavy element minerals.

 

The images above highlight a common sheet silicate mineral, known as a mica. Its platy texture is easily distinguishable on the adjacent minerals in the lower and upper left corners. This platy texture is the result of basal cleavage; minerals in the mica group have one cleavage plane and can be peeled into perfect thin sheets. Mica formations are generally associated with volcanoes and hydrothermal vents. Several of the brighter minerals scattered across the surface of the mica are calcite (CaCO3) crystals.

 

 

As expected the sediment from PC36 is highly siliciclastic (yellow), but contains very few quartz crystals, in PPL (right) and XPL (left).  Samples containing quartz are very stable and mature sediments. Therefore, this sample appears to be freshly deposited given its immature mineral assemblage. This supports the hypothesis that this sediment may have been transported from land by the Sepik River. This sample also contains a fair amount of opaque (red) minerals. Opaque minerals prevent light from passing through and commonly contain iron oxides and sulpihides. It is apparent that this sediment contains minimal skeletal debris as compared to previous sediments we have looked at. For comparison, see the previous post regarding PC23.

 

 

Deep-Sea Minerals

Jennifer Gonzales

Minerals are found in various shapes and sizes in the deep marine environment. Some minerals form naturally in the ocean through chemical processes, whereas others are transported to the sea from terrestrial environments by wind or water flow. We are going to take a look at a few minerals found at various coring sites in the New Guinea region. With the help of the EDX and knowledge of chemical and physical properties of minerals, at minimum it is possible to determine a minerals group.

 

At first glance mineral identification on a SEM seems like an impossible task. However, we can deduce that this mineral likely belongs to the amphibole group. This is largely due to its orthorhombic shape and 120° cleavage plane. In mineralogy, cleavage is used to describe how a crystal breaks on a particular plane. EDX point scan analysis indicates that the mineral contains approximately 3% Al, 7% Fe, 8% Mg, 9% Ca, 24% Si, and 41% O. Since the chemical composition of an amphibole is NaCa2(Mg,Fe,Al)5(Al,Si)8O22(OH)2, we can be certain that this mineral is an amphibole.

Is that really gold settling on top of marine sediment? Unfortunately, marine sediments are not a lucrative source of gold. What we are looking at here is pyrite (blue), known as fool’s gold. Pyrite has a chemical composition of FeS2 and its formation is largely dependent on sulfate availability in the ocean, where sulfate is converted by bacteria to hydrogen sulfide. Pyrite commonly forms perfect cubes, however in this case they are visualized as spherical aggregates of microcrystals, called framboids.

 

Heavy element minerals are indicators of mature, stable sediments. Since heavy minerals are often found concentrated in or around bedrock material, it is likely that this material is terrigenous. EDX microanalysis shows that this mineral contains approximately 17% Si, 50% Zr, and most of the remainder in oxygen. Based on the analysis, it can be inferred that this mineral displaying conchoidal fracture is a zircon (ZrSiO4) crystal.

Piston Core 23

Jennifer Gonzales

Since we have covered fundamental information regarding microfossil composition and structure, now we can focus on individual core sites and general geology of the New Guinea region. The science crew aboard the Roger Revelle (RR1313) collected a total of 54 cores near the Papua New Guinea margin. Sites PC23 and GC22 are the southeastern most sites, situated east of the Sepik Delta and west of the Manam Volcano. We will be focusing on piston core site 23 (PC23) collected at a water depth of 712 m and a subsurface depth of 431-432 cm.

Tectonically, the New Guinea region is complex due the formation of a subduction zone by rapid (geologically speaking) motion of the sea-floor. Subduction, indicated on the map as black lines with the teeth, is a geological process that occurs when two plate boundaries collide. As the two plates collide the denser slab sinks into the mantle and beneath the lighter slab, where temperature and pressure progressively increase with depth. Under these conditions trapped fluids such as seawater generates magma, which rises into the upper slab, and forms a volcano. In this case, the Indo-Australian Plate is moving northeastward but crashes into the Pacific Plate that is moving west with a slight north component. This vertical and horizontal displacement is geometrically classified as oblique-slip faulting, resulting in the formation of a volcanic chain.

 

Sediment at PC23 is particularly exciting because it is biologically and lithologically diverse. The image on the right displays several coccolithophores on top of flaky clay-sized particles. The image on the left features several coccolithophores that have situated on top of a “hole bearing” organism, known as foraminifera. A smaller foram fragment (orange) is visible in the lower right. Looking at sediments on a SEM is especially useful for end-member microanalysis. Mineral end-member identification is used to distinguish between biogenic, terrigenous, and hydrothermal sediments, all of  which may be altered when water masses mix and interact with rock.


 

 

An EDX detector paired with the SEM is able to generate data in the form of a spectra that displays known elements in the point scan by focusing a high-energy beam of particles into the sample. The output spectroscopy peaks for Spectrum 2 above indicates that calcium is present (peak intensity does not correspond with weight %), verifying that the microfossil analyzed is a coccolithophore.

 

 

 

In addition, scientists can analyze sediments underneath a petrographic microscope by preparing smear slides. A smear slide is a thin layer of unconsolidated sediment spread onto a glass slide. Petrographic analysis of a smear slide gives quantitative and limited compositional information with the ability to view the sample in plane-polarized light (PPL, top) and cross-polarized light (XPL, bottom). Under PPL, the sediment from PC23 appears to be clay-rich and contain minimal biogenics. However, under XPL we can see that the sediment is abundant in biogenics relative to the other particles. Coccolithophores (cluster in yellow) are calcite bearing organisms that are only visible in XPL. The dinoflagellate (red) and foraminifera (orange) are visible under both light conditions. The centered shard of vitric material (blue) is an indicator of hydrothermal activity and suggests volcanism. The combined use of a SEM and petrographic microscope make for strong tools to reconstruct ancient oceans.

 

 

 

 

Of quartz, that’s quartz!

Jennifer Gonzales

Quartz is one of the most common minerals within beach sand, and often serves as an indicator of weathering and erosion depending upon abundance and surface textures. The general public likely associates quartz with large transparent crystals (figure 1, left). Geologists often think of quartz as a common mineral within dikes, or a body of rocks/minerals that form in the pre-existing fractures of rock (figure 1, middle). On the other hand, sedimentologists are prone to thinking of quartz as semi-transparent and weather-resistant grains (figure 1, right).

Figure 1: (Left) Quartz crystals. (Middle) Dike comprised of a quartz vein. (Right) Quartz grains.

Quartz grains within sand must be identified before assessing weathering and erosional parameters. Figure 2 shows an SEM image of a sand sample from Moss Landing Beach at 55x magnification. To the untrained eye, distinguishing a quartz grain from feldspar, mica, magnetite, or other minerals may seem daunting. Many clues used to identify minerals, such as color, luster, streak, and hardness, cannot be employed here. But after a bit of searching, the grain in figure 3 appears promising due to a lack of both cleavage planes and large pits.

Figure 2: SEM image of a sand sample from Moss Landing Beach (magnification: 55x).

Figure 3: SEM image of a quartz grain from a sand sample taken at Moss Landing Beach (magnification: 100x).

Determining the elemental composition may help to confirm that the grain in figure 3 is quartz. Luckily, the SEM has an energy-dispersive x-ray spectrometer (EDX) that characterizes elemental composition. After only a few minutes of calibration and analysis the EDX provides results, suggesting that a point in the middle of the grain is 33.33% silicon and 66.67% oxygen. This supports the hypothesis that it is a quartz grain, given that the composition of quartz is SiO2.