Blog 9: Rapid weathering and erosion of the NZ Southern Alps

By Ignacio Jara and Helen Bostock

The debate about the role the Southern Alps (New Zealand) uplifting to offset global climate change has experienced a recent renewal with the publication of a couple of new articles. According to the Uplifting Theory, over geological time the tectonic erection of large mountain systems has been associated with lower global temperatures due to the enhanced consumption of atmospheric CO2 during the chemical weathering of terrestrial (silicate) material from the uplifting landscapes.

This year, Larsen et al (2014) reported new catchment erosion and soil production rates from the Southern Alps. Using Beryllium 10 (10Be) -a radioactive isotope produced by cosmic rays bombarding exposed mineral surfaces- the team found the highest measured rates of erosion and soil production (1). These chemical weathering rates (of about 2.5 mm/yr) are an order of magnitude higher than previously measured and higher than the suggested kinetically controlled limit. Moreover, the authors point out that soil weathering increases with erosion rate. The study also challenges the current dogma that high erosional environments are inefficient for soil production due to low residence time of key minerals. Even though denudation rates –the speed at which the landscape surface is eroding- exceed soil production in every catchment studied, the landscape is able to maintain a continuous soil mantle as a result of the dense vegetation cover (the main biotic soil producer in New Zealand due to the lack of native burrowing mammals). This seems to be a key point, as root expansion is an efficient mechanism for converting rock into soil, either by the physical breaking of bedrock or by increasing the concentration of organic acid and sub-surface CO2.

Thus, contrary to previous assumptions, this study demonstrates that rapidly uplifting mountains with dense vegetation cover are active weathering systems. The authors thus suggest that –

“These high weathering rates support the view that mountains play a key role in global-scale chemical weathering and thus have potentially important implications for the global carbon cycle”

Since weathering can only drive global climate trends if the erosion of silicate rocks removes enough CO2 from the atmosphere, the next step is to evaluate the amount of CO2 consumption resulting from the uplift of the Southern Alps.

This was the focus of another recent study in the Southern Alps using Calcium isotopes and Ca/Na ratios from rivers (2). They also show that weathering rates were an order of magnitude higher than the global mean and that the region has some of the highest chemical weathering rates in the world. However, they found that the majority of the weathering in non-glacial catchments was from carbonate rather than silicate rocks (a reaction that does not use atmospheric CO2), and thus the consumption of CO2 was no higher than the global average. Higher silicate weathering was only found in rivers downstream of glaciated regions. When these increased silicate weathering rates are extrapolated to all glaciated montane regions around the world, however, they only account for <1% of the global silicate weathering and global atmospheric CO2 consumption. Therefore they conclude that silicate weathering in uplifting mountain ranges like the Southern Alps has little effect on the global carbon cycle and long-term climate change.

Further work is clearly needed to better assess the weathering rates in tectonically active regions and then assess the role of mountain building and erosion in the global carbon cycle. Interestingly, enhancing weathering rates has recently been assessed as a potential geo-engineering strategy to offset increasing anthropogenic CO2 (3).

  1. Larsen et al., 2014. Rapid Soil Production and Weathering in the Southern Alps, New Zealand. Science 343, 637-640, doi:10.1126/science.1244908.
  2. Moore et al., 2013. Tracking the relationship between mountain uplift, silicate weathering, and long-term CO2 consumption with Ca isotopes: Southern Alps, New Zealand. Chemical Geology, 341, 110-127, doi:10.1016/j.chemgeo.2013.01.005.
  3. Hartmann, et al., 2013. Enhanced chemical weathering as a geoengineering strategy to reduce atmospheric carbon dioxide, supply nutrients, and mitigate ocean acidification. Reviews of geophysics, 51, 113-149.

Blog 8: Global volcanism linked to late-Quaternary deglaciations

By Ignacio A. Jara

It has been widely documented that volcanic aerosols can alter the radiative balance of the atmosphere, producing measurable temperature depressions following large explosive eruptions. Perhaps the most renowned of these cases is the eruption of the mount Tambora in Indonesia, which in 1815 caused a decline of 0.5°C in the Northern Hemisphere in what was known as “the year without a summer” (1).

But, what if this causal relationship is inverted and climate change affects the number of volcanic eruptions?

In 1979 Rampino et al., first used proxy data to argue that periods of climate cooling were associated with catastrophic volcanic events such as the latest Taupo eruption in New Zealand or the mega eruption of the Toba volcano in Indonesia (~70,000 cal yrs) (2). The authors boldly proposed that the changes in ice extent and sea level during glaciations resulted in sufficiently large variations in the Earth’s crustal stress to alter volcanic activity over longer time scales.

Since this pioneering study, a considerable number of publications have added local evidence supporting long-term climate variations as a driver for volcanic activity. However, solid evidence pointing to a definitive link between glacial cycles and volcanism at global scales as remained elusive.

An interesting article published in 2012 adds new insights into this theory. They analysed the timing of more than 400 tephra layers identified in marine sediment records around the Pacific “Ring of Fire” over the last 1 Myr (3). The spectral analysis of tephras deposited off South and Central America, Japan, the Philippines and the Southern Pacific islands reveals that periods of increased volcanic eruptions have a recurrence of 41 kyr, the exact periodicity of the Earth’s obliquity. Moreover, phase analysis indicates that peaks in volcanic eruptions lag behind minimum ice volume and maximum sea level by about 4 kyr.

Since obliquity has been recognized as one of the orbital pacemakers of the Pleistocene ice ages, these results indicate a direct link between orbital cycles, glacial/interglacial climate and global volcanism. The authors further suggest that this link could be mediated by surface pressure variations resulted from ice-ocean mass redistribution during periods of abrupt climate change. In this regard, volcanic eruptions along the edge of continental plates are expected to occur at greater frequency during periods of deglaciation, when ice retreat caused crustal pressure to decrease, lowering the compression of the rock overlying the magma chambers.

Undoubtedly these results are preliminary and much more work is required to better understand this phenomena. Even though the duration of the present-day warming trend is several orders of magnitude quicker than climate variations presented here, the interplay between climate change, ice extent and volcanism might be relevant in high-latitude regions with ice caps and active volcanic zones. As the atmosphere heats up at an unprecedented rate, the removal of large ice loads may contribute to the reactivation of volcanic complexes or the emergence of unknown volcanic systems.

References

1.            K. R. Briffa, P. D. Jones, F. H. Schweingruber, T. J. Osborn, (1998) Influence of volcanic eruptions on Northern Hemisphere summer temperature over the past 600 years. Nature 393, 450-455.
2.            M. R. Rampino, S. Self, R. W. Fairbridge, (1979) Can rapid climatic change cause volcanic eruptions? Science 206, 826-829.
3.            S. Kutterolf, M. Jegen, J. X. Mitrovica, T. Kwasnitschka, A. Freundt, P. J. Huybers, (2012) A detection of Milankovitch frequencies in global volcanic activity. Geology, (doi: 10.1130/g33419.1).

AUS2K meeting 26-27th June 2014 – Melbourne

We are pleased to announce that the third workshop of the PAGES Australasia2k Working Group will be held in Melbourne on 26-27 June 2014. This will be a joint meeting with the Australian Climate Change Science Program (ACCSP) project “Variability of Australian climate over the last 1000 years in coupled model simulations and proxy data”.

The topic of the workshop will be the challenges and opportunities in the growing fields of (a) climate reconstructions from proxy evidence, and (b) comparison with the PMIP3/CMIP5 Last Millennium climate model simulations, for the Australasian and wider Southern Hemisphere regions. The focus will be on the interaction between climate modellers, meteorologists, scientists developing proxy records, and how to strengthen ties to better understand the past (and future) behaviour of the climate system.

Day 1 (Thursday) will feature presentations from (1) Aus2k Working Group participants, and (2) palaeoclimate work being conducted by the ACCSP and other related work.

Day 2 (Friday) will be focussed on planning for Phase Two of Aus2k.

Further details, including a call for presentations, will follow during the coming weeks. For now, save these dates to ensure that you are able to attend.

When organising travel, please note that this workshop immediately precedes the AQUA Biennial Conference, which will take place in Mildura from 29 June to 4 July 2014.

We look forward to seeing you all in Melbourne!

Joelle, Drew, Steven, Nerilie, Russell, Ben, Jonathan, Heidi, Pandora and Jo – On behalf of the Aus2k Steering Committee and the ACCSP Project “Variability of Australian climate over the last 1000 years in coupled model simulations and proxy data”

Southern Hemisphere Westerly Wind SHAPE workshop summary

A small workshop was held on the 28-29th January 2014 in Auckland to discuss the Southern Hemisphere Westerly Winds and how they might have changed over time.

See the SHAPE webpage and scroll down to find a link to the summary pdf. from this recent workshop. Please get in touch with anyone listed if you are interested in being involved in any of the new or ongoing projects.

Blog 7: Dust deposition in the Southern Ocean

By Ignacio A. Jara and Helen Bostock

Terrestrial dust is important to the climate system not only because it may alter the solar radiative balance of the earth, but also because it supplies the oceans with key iron (Fe), a limiting micronutrient for phytoplankton productivity in the Southern Ocean. The “Iron Hypothesis” was first proposed by Martin (1990) [1], who suggested that changes in Fe supplied impact on the biological productivity in the Southern Ocean which, in turn, could influence the glacial-interglacial changes in atmospheric CO2.  However over the last 20 years scientists have struggled to find evidence to support this theory.

In the present-day Atlantic Southern Ocean an increase in terrestrial dust influx or volcanic aerosols has been linked to vast biological blooms. An increase in biological activity is associated with a high consumption of nutrients and transfers carbon back to the deep ocean. In 2009 an article published in Nature [2] explored past dust-climate interactions in this part of the Southern Ocean (42°S) by presenting an offshore dust record extending back to 4 million years.

This study of a long Ocean Drilling Program (ODP) core from the South Atlantic provided -with unprecedented detail- evidence for a consistently enhanced terrestrial dust influx during ice ages. It also showed a tight coupling with dust deposition in Antarctic ice cores over the last 0.8 million years, indicating that large areas of the Southern Ocean and Antarctica were affected by the glacial dust plume. According to the authors, the glacial dust was the result of an increased aridity of the eastern Patagonian plains (just upwind from the coring site), which increased the dust availability; as well as stronger and northward shifted Southern Hemisphere Westerly Winds, which enhanced the offshore transport of the Patagonian dust.

Has something similar occurred in larger Pacific sector of the Southern Ocean?

The dust record from a new set of cores collected by the RV Sonne 2010 from the Pacific Southern Ocean has recently been published in Science [3], and shows a similar pattern of variability in dust fluxes for this region over the last 1 million years [3]. The increased glacial dust deposition in the Pacific Southern Ocean is probably the result of enhanced supply from the Australian continent (with potential contributions from New Zealand), as modelling suggests this landmass is the main present-day dust source in the southern Pacific. However, geochemical fingerprinting of the dust will be required to determine the exact source.

Dust model

Figure: Modern terrestrial dust sources across the Southern Ocean based on modelling data (Lamy et al., 2014)

Interestingly, dust influxes during glaciations prior to 0.5 million years seem to have been significantly lower in the Pacific than in the Atlantic (Figure 2), something that may be explained by a less intense glacial desiccation of Australia due to its relatively northern position compared with Patagonia.

So has the increased dust/Fe translated into increased productivity?

There is evidence for some increase in productivity from opal flux, n-alkanes and barium concentrations in some of the cores north of the Polar Front. But there is a large reduction in productivity south of the Polar Front, resulting in an overall decrease in total biogenic opal production during the glacials in the South Pacific [4], and thus unlikely to draw-down the atmospheric CO2.

Dust and biology

Figure: Lithogenic and biogenic proxies from the subantarctic waters of the South Pacific (Lamy et al., 2014)

Modern iron fertilization experiments have also had mixed results; early experiments such as Ironex and SOIREE produced large blooms visible from space [5], while several subsequent experiments have not witnessed any major changes in phytoplankton concentrations. There is also the question of whether the blooms actually result in organic carbon being transferred to the deep ocean.

It is clear that more work is required to understand the link between dust, biological productivity and CO2 ventilation in the Southern Ocean .

References:

  • Martin, J.H., 1990. Glacial-interglacial CO2 change: The Iron Hypothesis. Paleoceanography, 5, 1-13.
  • Martinez-Garcia, A., et al., 2011. Southern Ocean dust-climate coupling over the past four million years. Nature 476, 312-315.
  • Lamy, F., et al., 2014. Increased dust deposition in the Pacific Southern Ocean during glacial periods. Science 343, 403-407.
  • Bradtmiller L., et al., 2009. Comparing glacial and Holocene opal fluxes in the Pacific sector of the Southern Ocean. Paleoceanography, 24, PA2214, doi:10.1029/2008PA001693
  • Boyd P.W. et al (2007) Iron enrichment experiments 1993-2005: synthesis and future directions. Science 315, 5812, 612-7.

Blog 6:Changes in radiocarbon surface reservoir ages in the SE Pacific

By Helen Bostock

Over the last decade the main theory to explain the changes in the atmospheric concentration of CO2 between glacials and interglacials, has primarily focussed on changes in the circulation of the Southern Ocean controlling the release of CO2 from the deep-ocean reservoir. However, there is still considerable debate about the path and timing of the CO2 release during the deglacial. A recent study on sediment cores from the SE Pacific has shed new light on this debate. Siani et al., (2013) found changes in the surface reservoir radiocarbon (14C) age, determined from the difference in the 14C age of planktic foraminifera compared to tephra ages in cores from the SE Pacific. The study found that periods of increased surface reservoir ages were coeval with the timing of upwelling events in the Southern Ocean and increases in atmospheric CO2 during the deglaciation. The increased upwelling of old, carbon-rich, deep-waters is supported by reductions in the difference between 14C benthic-planktic foram ages and the stable carbon isotopes (d13C), the latter of which are primarily controlled by biological production and respiration in the water column. They see three periods of upwelling initiating with a short pulse at the start of the deglaciation at 18.5 ka, then between 17.5 and 14.5 ka and finally between 12.5 to 11.5 ka.

The authors also recalculated the deep-water reservoir age for intermediate depths from the SE Pacific (De Pol Holz et al., 2010) and show that there is older Antarctic Intermediate Waters during the deglaciation. Thus providing a pathway for this old CO2 signal from the Southern Ocean.

This is also a critical study for showing that the surface 14C reservoir age can vary considerably overtime and if we do not adjust for these changing background reservoir age we cannot accurately compare paleoclimate records from the oceans with the ice and terrestrial records. Changes in the surface 14C reservoir age during the last deglacial have previously been suggested for the New Zealand region by comparing the 14C age of planktic foraminifera with local tephra (Sikes et al., 2000). But recent improved dating of some of the widely deposited tephra’s for example the original increased surface reservoir age around the time of the Waiohau Tephra no longer exists (Lowe et al., 2013). Perhaps it is time for a relook at the surface 14C reservoir ages around New Zealand before confidently comparing with other global records. This is however, trickier in regions where there is no widely deposited tephra layers that can act as chronostratigraphic timelines (e.g. Australia, South Africa).  One critical region where it will be very important is the Southern Ocean.

This work suggests we need to be careful comparing marine records dated using 14C and an assumed constant reservoir age with other paleoclimate records…..one of the main objectives of the INTIMATE and SHAPE projects.

References

De Pol Holz et al., 2010. No signature of abyssal carbon in intermediate waters off Chile during the deglaciation. Nature Geoscience,  3 , 192-195.

Lowe et a., 2013 Ages of 24 widespread tephras erupted since 30,000 years ago in New Zealand, with re-evaluation of the timing and palaeoclimatic implications of the Lateglacial cool episode recorded at Kaipo bog. Quaternary Science Reviews,74, 170-194.

Siani et al., 2013 Carbon isotope records reveal precise timing of enhanced Southern Ocean upwelling during the last deglaciation. Nature Communications, doi:10.1038/ncomms3758.

Sikes et al., 2000. Old radiocarbon ages in the southwest Pacific Ocean during the last glacial period and deglaciation. Nature, 405, 555-559.

Membership

Memberships are well and truly due and you can pay these easily online.

It is easy, so please get yourself signed up and encourage your colleagues, students and friends. This is a great time to sign new people up, as anyone that joins AQUA now will automatically have their membership extended right through to 28 February 2015. Members also have reduced registration fees at the upcoming AQUA Mildura 2014 – “Back to the Core” conference.

If, in the rare case you do have an issue with the online system, please contact our treasurer, Steven Phipps (treasurer@aqua.org.au).

Blog 5: Precipitation and human occupation changes in arid environments

Written by Ignacio Jara, Victoria University Wellington.

While the abrupt climate transitions of the glacial termination and the establishment of the present-day modes of climate variation during the Holocene seems to be well characterized in proxy records around the Southern Hemisphere, there is still sparse evidence about the impact of those changes on the migration, settlement and cultural development of ancient human populations.

A recent approach to this subject includes to compile the radiocarbon dates from archaeological sites as a proxy for human density. The underlying assumption is that period of increasing human presence should be reflected as a higher number of sites with dates falling within such the time interval. Whereas biases and limitations may be reduced by using larger radiocarbon data set, the main advantage of this method is that it provides a chronology of human population that can be directly compared with climate proxies. Two newly-published articles use this technique to uncover human responses to late Quaternary climate events.

The first of them combines a data set of more than 5,000 radiocarbon dates from all around the Australian continent with geospatial modelling (1). The main goal is to investigate the association between contractions and expansions of aboriginal population and climate instability during the terminal Pleistocene.

In Australia, cold intervals such as the Last Glacial Maximum (LGM; 23,000-18,000 years ago) have been associated with increased aridity and the expansion of grassland and non-vegetated landscapes (see blog number 3). What it is new from this study is that human population seems to have experienced a significant reduction during the LGM, as indicated by a decrease in the total number of radiocarbon dates of that age. Moreover, archaeological sites show what appears to be an interruption of elaborated cultural behaviour common prior to the LGM such as rock art, ritual burials and coloured ornamentation. These types of cultural expressions are only resumed in the early part of the Holocene Period.

The hyper arid Atacama Desert in northern Chile is also an interesting region to investigate relationship between precipitation and population changes since it is a natural passageway to southern South America, an area where the oldest evidence of human settlement in the continent.

The most intense pulses of occupation in Atacama seem to be coeval  with late Quaternary rainfall events in the Central Andes. For instance, a well preserved archaeological site published this year shows continuous human occupation in the rainless part of the dessert during one of the last of these rainfall pulses between 12,700-9700 year ago (2). Under increasing moisture, the previously plantless landscape would have been scattered by small wetlands and woodlands, becoming oases for migrating populations and a bio-geographic corridor connecting the dessert with the more humid biomes of the south.

But perhaps more tantalizing is the cultural developments associated with increasing precipitation between 7,000-4,000 years ago in Atacama. Using a data set of more than 400 radiocarbon dates, a noteworthy scientific contribution published last year correlates increased rainfall with a notable increment in human density (3). The authors point out that more water availability and larger groups of people led to a period of rapid technological and cultural innovation which resulted in the emergence of the oldest examples of artificial mummification in the world (as early as 7,000 year ago).

Atacama mummy
Increasing water availability could have been one of
the environmental drivers behind the oldest known examples of artificial mummification in the Atacama Desert between 8,000 and 7,000 years ago. Photograph from Marquett et al. 2013.

Although arid environments can restrict human settlement and migration, they also provide extraordinary conditions for the conservation of archaeological sites which might be a great advantage for future studies that address climate-human relationship during the past. What makes the results meaningful is the possibility that natural conditions for human preservation became a distinctive and influential cultural trait upon early human groups. The preservation of the soul with the body after death is a common belief among traditional societies. So the lack of decomposition is not just a relevant issue for Quaternarists.

References

  1. A. N. Williams, S. Ulm, A. R. Cook, M. C. Langley, M. Collard, Journal of Archaeological Science 40, 4612 (2013).
  2. C. Latorre et al., Quaternary Science Reviews 77, 19 (2013).
  3. P. A. Marquet et al., Proceedings of the National Academy of Sciences 109, 14754 (September 11, 2012, 2012).

Blog 4: Long-term orbital changes and glaciations in the Southern South America

Written by Ignacio Jara, Victoria University Wellington

Although the long-term changes in solar insolation between Northern Hemisphere (NH) and the Southern Hemisphere (SH) are in anti-phase, marine and ice core records from both hemispheres show highly synchronous glacial/interglacial cycles over the last 800 kyr. This disparity between the insolation and the paleo-records is probably the most important caveat concerning Milankovitch’s theory of orbital parameters controlling Earth’s climate. Particularly puzzling for the SH is that glacial/interglacial transitions follow the NH summer insolation, and therefore deglaciations in southern latitudes occur under decreasing local summer insolation1. In order to resolve this apparent conflict it is necessary to develop new detailed glacial chronologies which are able to be compared with other glacial and climate reconstructions from both hemispheres.

Southern South America has the most extensive ice sheets of the Southern Hemisphere outside of Antarctica, and thus it is not surprising this area boasts outstanding geomorphologic evidence of late Quaternary glacial fluctuations. Yet, the timing of these fluctuations has remained more or less equivocal until recently. Two newly-published glacial reconstructions from this part of the world provide new interesting insights into this topic.

Firstly, a detailed geomorphological map of the Torres del Paine area (51°S) has been produced. This is a region with extensive glaciers and massive moraine belts extending up to 35 km from present-day ice margins2. The boulder exposure 10Be dates indicate a major glacial advance culminating at 14,200 cal yr BP, while basal 14C dates from peat sections embedded in the moraine complexes indicate that ice sheet remained extended until 12,500 cal yr BP.

This chronology is supported by a more recent publication of surface exposure and radiocarbon ages from moraine complexes in Tierra de Fuego (54°S), the southern-most tip of South America and a region whose geomorphology was previously poorly mapped and dated3. The authors attribute the lack of early-Holocene moraines as evidence for dry and warm conditions during this period and suggest that the glaciers may have reached near present-day positions as early as 11,200 cal yr BP.

Overall, these two reconstructions show clear evidence for a net glacial retreat between 16,000-11,000 cal yr BP, a period of decreasing SH summer insolation. Likewise, the Tierra del Fuego reconstruction shows the absence of sustained glacial retreat during the Holocene, a period of increasing summer SH insolation. Hence, these studies are consistent with the orbital/climate paradox, suggesting that the SH summer insolation does not control (at least not directly) the ice fluctuations on the Southern American continent.

So what caused this change?

Apart from the NH summer insolation, it has been argued that other orbital parameters such as the SH summer duration and SH spring insolation could be important drivers1. The remarkable detail in the glacial reconstructions presented in these two publications may provide other clues. The ice advances during the Antarctic Cold Reversal (ACR; 14,500-12,900 cal yr BP) are observed at both the Torres del Paine and Tierra del Fuego regions, suggesting a strong Antarctic influence. Moreover, ACR glacial advances have also been recorded in the Southern Alps of New Zealand4, suggesting a zonally synchronous response across the SH, which may point to the Southern Westerly Winds and/or the Southern Ocean as important players.

Taken together, these recent publications are important contributions to better constrain the timing and forces affecting the SH glacial history over the last 16,000 years. They also highlight the potential of Southern South America for glacial reconstructions, and show the importance of glacial geomorphology studies for late Quaternary global climate reconstructions.

S Am glaciers

Glacial retreat between 16,000-11,000 cal yr BP in Tierra del Fuego occurs under decreasing Southern Hemisphere summer insolation. ACR, YD and LIA denote Antarctic Cold Reversal, Younger Dryas and the Little Ice Age respectively. Pink circles and green squares indicate 10Be dates, while red triangles indicate radiocarbon dates from the different sites from Menounos et al. (2013).

References   

1.            Huybers, P. & Denton, G. Antarctic temperature at orbital timescales controlled by local summer duration. Nature Geosci 1, 787-792 (2008).

2.            García, J.L., et al. Glacier expansion in southern Patagonia throughout the Antarctic cold reversal. Geology 40, 859-862 (2012).

3.            Menounos, B., et al. Latest Pleistocene and Holocene glacier fluctuations in southernmost Tierra del Fuego, Argentina. Quaternary Science Reviews 77, 70-79 (2013).

4.            Putnam, A.E., et al. Glacier advance in southern middle-latitudes during the Antarctic Cold Reversal. Nature Geosci 3, 700-704 (2010).

Inaugural SHAPE workshop Sept 2013

The first SHAPE workshop was held at GNS Science on the 16-17th September 2013. A summary of the talks and discussion at the workshop can be found on the SHAPE project page.

Lots of ideas were discussed and it was clear that there is still momentum continuing on from the Aus-INTIMATE project. There is a long list of possible topics towards the end of the summary with names of the lead on each (mostly people at the meeting). If you think you have something to contribute to any of these projects and would like to get involved then get in touch with the lead. With such a short time frame between now and the next INQUA in Japan in 2015, this long list of topics is clearly ambitious – so please help us out by getting involved.