Erin L. McClymont

Subpolar Link to the Emergence of the Modern Equatorial Pacific Cold Tongue

Birth of the Cool Over the past 4 million years or so, tropical sea surface temperatures have experienced a cooling trend (see the Perspective by Philander). Herbert et al. (p. 1530) analyzed sea surface temperature records of the past 3.5 million years from low-latitude sites spanning the worlds major ocean basins in order to determine the timing and magnitude of the cooling that has accompanied the intensification of Northern Hemisphere ice ages since the Pliocene. Martínez-Garcia et al. (p. 1550) found that the enigmatic eastern equatorial Pacific cold tongue, a feature one might not expect to find in such a warm region receiving so much sunlight, first appeared between 1.8 and 1.2 million years ago. Its appearance was probably in response to a general shrinking of the tropical warm water pool caused by general climate cooling driven by changes in Earths orbit. The eastern Pacific Ocean cold tongue appeared as Earth’s climate cooled and subpolar waters expanded in the Pleistocene. The cold upwelling “tongue” of the eastern equatorial Pacific is a central energetic feature of the ocean, dominating both the mean state and temporal variability of climate in the tropics and beyond. Recent evidence for the development of the modern cold tongue during the Pliocene-Pleistocene transition has been explained as the result of extratropical cooling that drove a shoaling of the thermocline. We have found that the sub-Antarctic and sub-Arctic regions underwent substantial cooling nearly synchronous to the cold tongue development, thereby providing support for this hypothesis. In addition, we show that sub-Antarctic climate changed in its response to Earth’s orbital variations, from a subtropical to a subpolar pattern, as expected if cooling shrank the warm-water sphere of the ocean and thus contracted the subtropical gyres.

Links between the onset of modern Walker circulation and the mid-Pleistocene climate transition

Sea-surface temperatures (SSTs) reconstructed from Ocean Drilling Program Sites 806 and 849 in the western and eastern equatorial Pacific provide a history of the strength of the Walker circulation between 1.5 and 0.5 Ma. A progressive cooling in the eastern equatorial Pacific is paralleled by intensification of the equatorial zonal SST gradient that began 1.17 Ma, prior to the expansion of global ice volume associated with the mid-Pleistocene climate transition. The SSTs in the eastern equatorial Pacific and the zonal gradient reached modern values by 0.9 Ma. We propose that this onset and intensification of the modern Walker circulation reduced heat flux, but increased moisture transport to high latitudes, leading to the development of more extensive ice sheets and the shift toward the 100 ka world.

On the identification of a Pliocene time slice for data–model comparison

The characteristics of the mid-Pliocene warm period (mPWP: 3.264–3.025 Ma BP) have been examined using geological proxies and climate models. While there is agreement between models and data, details of regional climate differ. Uncertainties in prescribed forcings and in proxy data limit the utility of the interval to understand the dynamics of a warmer than present climate or evaluate models. This uncertainty comes, in part, from the reconstruction of a time slab rather than a time slice, where forcings required by climate models can be more adequately constrained. Here, we describe the rationale and approach for identifying a time slice(s) for Pliocene environmental reconstruction. A time slice centred on 3.205 Ma BP (3.204–3.207 Ma BP) has been identified as a priority for investigation. It is a warm interval characterized by a negative benthic oxygen isotope excursion (0.21–0.23‰) centred on marine isotope stage KM5c (KM5.3). It occurred during a period of orbital forcing that was very similar to present day. Climate model simulations indicate that proxy temperature estimates are unlikely to be significantly affected by orbital forcing for at least a precession cycle centred on the time slice, with the North Atlantic potentially being an important exception.

Expansion of subarctic water masses in the North Atlantic and Pacific oceans and implications for mid-Pleistocene ice sheet growth

[1] Past surface ocean circulation changes associated with the mid-Pleistocene transition, 0.9–0.6 Ma, were reconstructed in the northern North Atlantic (ODP 983) and the northwest Pacific (ODP 882), using proxies for subarctic/subpolar water mass distributions (%C37:4 alkenone) and sea surface temperature (U37 ). Both sites experienced a secular expansion of subarctic waters from � 1.15 Ma, spanning both glacial and interglacial intervals. After 0.9 Ma, low %C37:4 at Site 983 records a northward retreat of subarctic waters during interglacials in the Atlantic, while continued high glacial %C37:4 indicate extensive subarctic waters during glacial maxima associated with the development of the larger late Pleistocene ice sheets. In contrast, a secular decline in %C37:4 occurred at Site 882 from 0.9 to 0.5 Ma, marking a more gradual retreat of subarctic conditions in the Pacific. It is proposed that the expansion of subarctic waters between 1.15 and 0.9 Ma exerted negative feedbacks to the moisture supply to the ice sheet source regions and may account for the apparent delayed ice sheet response to atmosphere-ocean circulation changes associated with the mid-Pleistocene transition that began as early as 1.2 Ma.

Mid-Pleistocene climate transition drives net mass loss from rapidly uplifting St. Elias Mountains, Alaska.

Significance In coastal Alaska and the St. Elias orogen, over the past 1.2 million years, mass flux leaving the mountains due to glacial erosion exceeds the plate tectonic input. This finding underscores the power of climate in driving erosion rates, potential feedback mechanisms linking climate, erosion, and tectonics, and the complex nature of climate−tectonic coupling in transient responses toward longer-term dynamic equilibration of landscapes with ever-changing environments. Erosion, sediment production, and routing on a tectonically active continental margin reflect both tectonic and climatic processes; partitioning the relative importance of these processes remains controversial. Gulf of Alaska contains a preserved sedimentary record of the Yakutat Terrane collision with North America. Because tectonic convergence in the coastal St. Elias orogen has been roughly constant for 6 My, variations in its eroded sediments preserved in the offshore Surveyor Fan constrain a budget of tectonic material influx, erosion, and sediment output. Seismically imaged sediment volumes calibrated with chronologies derived from Integrated Ocean Drilling Program boreholes show that erosion accelerated in response to Northern Hemisphere glacial intensification (∼2.7 Ma) and that the 900-km-long Surveyor Channel inception appears to correlate with this event. However, tectonic influx exceeded integrated sediment efflux over the interval 2.8–1.2 Ma. Volumetric erosion accelerated following the onset of quasi-periodic (∼100-ky) glacial cycles in the mid-Pleistocene climate transition (1.2–0.7 Ma). Since then, erosion and transport of material out of the orogen has outpaced tectonic influx by 50–80%. Such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out-of-sequence fault patterns. The 1.2-My mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response (millions of years). The St. Elias Range provides a key example of how active orogenic systems respond to transient mass fluxes, and of the possible influence of climate-driven erosive processes that diverge from equilibrium on the million-year scale.

The disappearance of Sphagnum imbricatum from Butterburn Flow, UK

The disappearance of the previously abundant moss species Sphagnum imbricatum has been investigated at Butterburn Flow, northern England, using organic geochemical, elemental, macrofossil, pollen and testate amoebae analyses. Variations in the assemblage of peat-forming plants were tracked using the macrofossil distributions as well as the relative chain lengths of n-alkanes and concentrations of 5-n-alkylresorcinols and triterpenols. No significant changes to the vegetation assemblage could be detected prior to the loss of S. imbricatum. Variations in water depth were reconstructed using a testate amoebae transfer function and inferred qualitatively using bulk elemental composition and biomarkers for changing redox conditions in the bog subsurface: the degree of isomerization in the C31 hopanes, and the concentrations of bishomohopanol and archaeol. Pollen analysis reconstructed the landscape surrounding the mire and revealed evidence for human disturbance. The results suggest that bog surface wetness increased with the transition from Sphagnum imbricatum to Sphagnum magellanicum, but the increase was not large and S. imbricatum had previously survived similar periods of wetness. However, the loss of S. imbricatum coincides with increasing human disturbance surrounding the bog, which may have altered nutrient inputs to the bog surface from agriculturally derived dust, to the detriment of S. imbricatum but to the benefit of S. magellanicum and Eriophorum vaginatum. It is proposed here that the stresses imposed by the combination of changing nutrient inputs and a rapidly rising water-table drove the disappearance of S. imbricatum from Butterburn Flow at c. cal. AD 1300.

Palaeoclimate reconstructions reveal a strong link between El Nino-Southern Oscillation and Tropical Pacific mean state

The El Niño-Southern Oscillation (ENSO) is one of the most important components of the global climate system, but its potential response to an anthropogenic increase in atmospheric CO2 remains largely unknown. One of the major limitations in ENSO prediction is our poor understanding of the relationship between ENSO variability and long-term changes in Tropical Pacific oceanography. Here we investigate this relationship using palaeorecords derived from the geochemistry of planktonic foraminifera. Our results indicate a strong negative correlation between ENSO variability and zonal gradient of sea-surface temperatures across the Tropical Pacific during the last 22 ky. This strong correlation implies a mechanistic link that tightly couples zonal sea-surface temperature gradient and ENSO variability during large climate changes and provides a unique insight into potential ENSO evolution in the future by suggesting enhanced ENSO variability under a global warming scenario.

Silicic acid biogeochemistry in the Gulf of California: Insights from sedimentary Si isotopes

reflects transient iron limitation. Our new d 30 Si record from the Guaymas Basin shows dramatic variations at millennial timescales. Low d 30 Si values synchronous with Heinrich events are interpreted as resulting from the decline in Si(OH)4 utilization at times of decreased upwelling strength, while nearly complete Si(OH)4 utilization was observed at times of invigorated upwelling and increased opal burial during the Holocene, the Bolling-Allerod and the last glacial period. We attribute the complete utilization of Si(OH)4 to the occurrence of transient Fe limitation at these times. Our study highlights the importance of Fe limitation on Si and C cycling in coastal upwelling regions and suggests that upwelling dynamics, in combination with Fe availability, have the potential to modulate marine Si distribution and opal burial even at short timescales. Citation: Pichevin, L., R. S. Ganeshram, B. C. Reynolds, F. Prahl, T. F. Pedersen, R. Thunell, and E. L. McClymont (2012), Silicic acid biogeochemistry in the Gulf of California: Insights from sedimentary Si isotopes, Paleoceanography, 27, PA2201,

Atlantic overturning circulation and Agulhas leakage influences on southeast Atlantic upper ocean hydrography during marine isotope stage 11

Climate dynamics during the marine isotope stage (MIS) 11 interglacial may provide information about how the climate system will evolve under the conditions of low-amplitude orbital forcing that are also found during the late Holocene. New stable isotope and alkenone data are presented from southeast Atlantic Ocean Drilling Program Site 1085, providing detailed information on interglacial climate evolution and the impacts of Atlantic meridional overturning circulation (MOC) and Agulhas leakage on the regional upper ocean hydrography. The data suggest that although warm surface ocean conditions were maintained at approximate Holocene levels for 40,000 years during MIS 11, subsurface temperature and salinity recorded by deeper-dwelling planktonic foraminifera species were maintained at their highest values for only 7000–8000 years. Surface water temperature and salinity data suggest that the interocean exchange of warm, salty waters into the southeast Atlantic Ocean was directly related to changes in the activity of the MOC during the study interval. Specifically, transient regional warming events during periods of weakened overturning circulation may have been amplified by the continuous interocean exchange of warm, salty Indian Ocean waters that primed the MOC for abrupt resumptions into a vigorous mode of operation. Conversely, a peak in interocean exchange at the end of the MIS 11 interglacial optimum may reflect enhanced trade wind forcing of surface waters whose export to the North Atlantic Ocean could have contributed to renewed ice sheet buildup during the MIS 11 to 10 glacial inception.

Chapter Eleven Biomarkers as Paleoceanographic Proxies

Publisher Summary In palaeoceanography the term biomarker is used to name organic molecules found in sediments, initially produced by a variety of organisms either on land or in the aquatic environment. A key characteristic of biomarkers is that after their biosynthesis, and the death of the source organisms, they survive deposition to sediments in a recognizable form in terms of their original structure and sterical configuration (that is, spatial distribution of the atoms). They can thus be considered chemical fossils. The usefulness of organic components as palaeoproxies largely depends on their resilience to early degradation processes during sedimentation and after incorporation into the sediment. The most common biomarkers used as climate proxies belong to a few compound types, namely C 37 alkenones and n -alkyl lipids. The key factors that make them suitable as proxies are that they are extremely common in sediments, their sources are identified, and their study provides information on key environmental variables. Biomarkers may be transported to sediments as part of the remains of the original organism (for example, leaf debris, marine snow), its digested remains (for example, faecal pellets), or adsorbed to mineral particles (that is, ballast minerals: silicate and carbonate biominerals, and dust), which eventually settle on the ocean bottom through gravity. The chapter describes the various approaches at distinct stages in their development and application.