It's a Cool, Cool Miocene World - Part II

Well folks, I hope the suspense of learning about Middle to Late Miocene climate and tectonics in Africa hasn’t kept anyone up at night. With final exams, conferences and a new puppy (fun!) the blog had to take a back seat for a bit. But now, with the lazy (ha!) days of summer upon us, let’s continue with the…

TOP FIVE Incredibly Fascinating Facts (Theories? Speculations?) about Middle to Late Miocene Climate Change in Africa (and Worldwide too!)

#4. Evolution of ocean currents along the margin of Southwest Africa

Several deep-sea drilling sites have been investigated in the South Atlantic in an effort to characterize evolving ocean current circulation and latitudinal temperature gradients. For example, the following figure is from Robert et al. (2005) and shows the location of DSDP sites off the coast of SW Africa.



As Mel commented after the last post, there are some interesting links between ocean currents in this region and climate and sedimentation along the coast. The present-day Benguela current dominates ocean circulation and consists of a wide, cold layer approximately 80m thick that flows northward along the southwest African coastline. A counter-current flows southward at approximately 200-600m water depth and transports oxygen-depleted tropical waters. Beneath these currents the Antarctic Intermediate Water spreads nutrient-rich waters through the Cape Basin. To read more about the interaction of these currents, check out Robert et al. (2005) and Westerhold et al. (2005).

Deep marine climate proxy data from DSDP sites provide information about the evolution of these currents through time. For example, Antarctic nannofossils have been recovered from Early Oligocene sediments along the Walvis Ridge indicating northward ocean current intensification during this period. Upwelling along the southwest African coast likely began in the Late Miocene during formation of Antarctic ice sheets and caused increasing aridity along the Namib coastline. This is a really neat example of the interaction between deep ocean processes and climate/sedimentation on the adjacent continent!

It's a Cool, Cool Miocene World - Part I

One of the most interesting aspects of my research is learning about the interactions between climate, tectonics and sedimentation through time. Recently, I’ve been reading up on the sedimentary record of Middle to Late Miocene climate changes, particularly as recorded in West Africa. Folks, this is a fascinating topic! If you’re wondering what I mean, stick with me for a couple posts as I reveal...

TOP FIVE Incredibly Fascinating Facts (Theories? Speculations?) About Middle to Late Miocene Climate Change in Africa (and Worldwide too!)

#5. Stepwise cooling in the Cenozoic and the Middle Miocene ‘jump’.



In terms of Cenozoic cooling, the Middle Miocene was characterized by a significant ‘jump’ in cooling around 14.8 – 14.1 Ma. The figure above comes from Miller et al. (1987) and shows the general cooling trend beginning in the Early Eocene and proceeding, somewhat stepwise, to the present. This particular diagram is based on oxygen isotopes (top horizontal axis) measured from benthic foraminifera in the Atlantic Ocean, which serve as a proxy for deep ocean temperatures (bottom horizontal axis).

Next up: What are some potential causes for these climatic jumps?

Reference:
(Sorry, I couldn't find a good online source for this one.)
Miller, K.G., Fairbanks, R.G. and Mountain, G.S., 1987, Tertiary oxygen isotope synthesis, sea level history, and continental margin erosion. Paleooceanography, v. 2, p . 1-19.

UPDATE: Hat tip to Dmonte!
Miller's papers are available as PDFs at: http://geology.rutgers.edu/miller.shtml

Ancient plate tectonics on Mars? Hmmm...

As a contribution to this edition of the Accretionary Wedge, I’d like to scrape off the following: The hemispherical dichotomy and the idea of ancient Martian plate tectonics make me go hmmm….



This is a topographic map of Mars created using data from the NASA Mars Orbital Laser Altimeter (MOLA). The dark blues and cool colors represent topographic lows; the reds and warm colors represent topographic highs. The ‘hemispherical dichotomy’ refers to the difference between the topography of the northern 1/3 of the planet and the southern 2/3 of the planet. The northern hemisphere of Mars sits approximately 2-3 km topographically lower than the southern hemisphere, and the south pole is estimated to be ~6 km higher than the north pole.

In addition, the surface geology of each hemisphere is radically different. Craters and terrain heavily mark the southern hemisphere, but the northern lowlands are relatively smooth and flat. If crater density is used as an indicator of absolute age (i.e., crater counts), then the southern hemisphere may be ancient crust formed during the accretion of Mars and the northern hemisphere may be much younger.

It’s generally agreed that Mars lacks a self-sustaining magnetic field and is not tectonically active, BUT… did plate tectonic processes operate on Mars in the past? I submit, for your enjoyment and wonder (hmmm…) the following observations:



1.) The Martian surface is characterized by both extensional features (e.g., grabens and rifts) and contractional features (e.g., wrinkle ridges), but evidence of strike-slip movement is rare. To the right is a photo of a heart-shaped graben taken by the Mars Global Surveyor Orbital Camera. Are these types of features telling us something about Martian tectonic history?

2.) Numerous volcanic features on the Martian surface span a wide range of geologic ages, which are estimated using cross-cutting relationships and crater counts. Studies of the Tharsis region, a volcanic plateau with several shield volcanoes, provide information on the petrology and structure of the Martian interior. For example, Olympus Mons (photo below), a shield volcano twice the size of Mauna Loa in Hawaii is characterized by ancient voluminous basalt flows. What does this type of feature imply about isostatic compensation and the interior structure of Mars?



3.) Finally, the Mars Global Surveyor made repeated low-altitude passes through the martian atmosphere to circularize the spacecraft’s orbit using atmospheric drag. Magnetic data was collected every time the spacecraft dropped below 200km; the closest approach was at 101km. From this data, Connerney et al. (1999) present a martian magnetic field map.


Red and blue coloring represent positive and negative radial field measurements, and correlation between adjacent MGS tracks is possible. Are these magnetic stripes on the martian crust? Are martian anomalies a result of tectonism?

Hmmm…

Statically-charged saltating sand

Wind is the dominant sediment transport mechanism in aeolian (desert) systems. Who hasn’t seen beautiful photos of migrating dunes or reddish sandstorms? If you’ve missed out, here’s a pretty one from Conception Bay on the Namib coastline, courtesy of NASA. The left side of the image is the relatively flat strand plain, where dunes are generally subparallel to the beach. The complex dune patterns on the right side are part of the Namib dune sea.



Particle movement in the aeolian environment is pretty interesting. Sand-sized grains can move via suspension, traction or saltation, all of which are essentially functions of settling velocities vs. shear velocities. (e.g., suspension of grains occurs when shear velocities are much greater than settling velocities).

One parameter that hasn’t been extensively studied is how static electricity influences particle movement. How do electrostatic forces effect saltation and the movement of grains along a surface? There’s a neat new paper in Physical Review Letters by Kok and Renno (2008)** that describes this interaction (also, there’s a short review in Nature). Basically, the authors found that charged particles tended to levitate above a surface or jump considerable distances, thus making it easier for them to become airborne than previously thought. Exactly how this type of grain interaction influences sand flow or bedform development is a pretty interesting question.

**Warning – There’s some heavy duty math/modeling going on in this paper so make sure your coffee cup is FULL.

Gender and the geosciences

There have been some really good posts/responses about gender (dis)parity in the earth sciences (Eric, Kim, Chuck, etc.), stemming from discussion of an article in the new Nature Geosciences journal (Holmes et al., 2008). I haven’t read the article, but this has been an ongoing discussion on a local women-in-science-and-engineering listserv that I subscribe to. In this case, the input has come from women in all branches of science and from all levels of post-undergraduate education (i.e., M.S., Ph.D., post-doctoral, professors, etc.) It’s interesting to compare these responses with the information from the Holmes et al. (2008) article (courtesy of Eric), and I thought I’d share some of my observations, humble opinions, and a final note on my own experiences thus far.

First, the discussion on the listserv began following the publication of a similar article, in NIH news, referring to a study in the November issue of EMBO Reports (another Nature-related publication). At the NIH, 29% of tenure-track principle investigators (equivalent to an assistant professor) are women, and only 19% are tenured principle investigators (equivalent to a full professor). It appears that this decrease in women working at full professor level is not limited to the geosciences, although the percentages are significantly lower (14% = assistant prof, 8% = full professor). This article concludes that there are two primary reasons for this disparity: family responsibilities and self-confidence. In my view, these are similar to the ‘structural’ problems and ‘intrinsic female’ problems investigated in the Nature Geosciences article. [Eric over at the Dynamic Earth has renamed ‘intrinsic female attributes’ womany-ness, which is fantastic.]

Now, in general, this local list-serv has approximately 10 posts a month, usually information about recent lectures, upcoming events, etc. When the moderator posted this article, well… all hell broke loose listserv-wise. There were 32 posts over 4 days. Many of the responses were from women who had struggled with the ‘structural’ problems of balancing child-care, family responsibilities, etc., and how they had managed these tasks. Others weighed in with information about how they had found mentors and how valuable mentoring relationships were. BUT, NOT ONE replied with what could be characterized as problems with ‘intrinsic female attributes’ (e.g., lack of self-confidence, womany-ness).

If we compare the list-serv responses with the male vs. female responses in the Nature Geosciences article, things get pretty interesting. Only 1 out of 40 women in the article thought that ‘intrinsic female attributes’ may keep women from excelling in geoscience academia. In shocking agreement, none of the 32 listserv respondents suggested womany-ness was ever a problem. Now, I realize that these are two completely separate groups; on one hand we have 40 male and female academic geoscientists polled for the Nature article, and on the other we have women scientists and engineers on a local academic listserv. Still, it’s a pretty neat correlation.

As for my own experiences, I hadn’t thought about it much before, but here are some observations made in light of this discussion:

University #1 – I received an undergrad degree from a small geology department with 2 full professors (both male), and 1 part-time professor (female, working on her PhD).

University #2 – I did grad school at a larger university with 22 full-time faculty, 2 of which were female (never had a class or did research with either).

University #3 – I am working towards a doctorate at a mid-size university with 15 full-time faculty, 1 of which is female and whom I have taken a class with.

Obviously, this dataset is highly biased toward my own individual experiences, but nonetheless it illustrates the 8% problem very nicely. It is also interesting to note that none of these women have children (see Julia’s post and responses); thus, after almost 9 years spent at various universities, I have yet to see a woman balancing children and a career in geoscience academia. This has never bothered me, but it’s an interesting observation.

Ogooue Watershed in Gabon

I’m very interested in learning more about drainage basin systems around the world. While drifting about (rather aimlessly) on Google Earth, I came upon this fantastic satellite image of the Ogooue watershed in equatorial West Africa. The image was taken by the European Space Agency’s Envisat radar sensor, which is unaffected by clouds or other optical obstructions.

One of the neat things about the West African coast is the transition from huge drainage systems in the equatorial region (e.g., Niger Delta, Congo River) to smaller, localized drainage basins in the southern region. I hope to post more about this soon; if you’re interested, check out these great papers by Seranne and Anka (2005) and Lavier et al. (2001).

Introduction

Many thanks to Mel for introducing me to the sedimentary blogging world! I've just spent the better part of a Friday night reading several blogs, and I immediately had to have my very own. Yes, this is an impulse blog. And if I fade away after only a couple weeks...well, consider yourself warned.

I also just spent an incredible amount of time trying to select an appropriate name for this blog. See, I happen to really love geo-puns, and the idea of trying to pick JUST ONE for a title is nearly impossible. So, I had to get strict with myself, buckle down, and set some criteria for a blog title.

1. Must be timeless and classy.
When you deal with deep time, you want things that will last. Okay, I realize this is just a blog, and an impulse blog at that, but still...

2. Must be based in reality.
My first impulse was to go with "Paragneiss Legs", but I would never consider myself a metamorphic petrologist, let alone one with rockin legs. Damn.

So, I named this blog Ten Million Years of Solitude. Timeless and classy, based in reality. And yes, also as a bit of a tribute to the brilliant author Gabriel Garcia Marquez. And to Late Miocene sedimentary rocks worldwide - this blog's for you!