Displays

When the Earth warms, the high latitudes often warm more than the low latitudes, a phenomenon commonly known as high latitude amplification. Although high latitude amplification has been observed by both climate data and models, the trajectory of high latitude amplification in our future changing climate is uncertain. Pacific-wide reconstructions of sea surface temperature variability from past climates are important for establishing the historical records of high latitude amplification. Multiple extratropical temperature records have been established for the past 10 million years (Myr). However, it is debated whether the warmest end member, the Western Pacific Warm Pool (WPWP), warmed during the late Miocene (~12 to 5 million years ago, Ma) and Pliocene (5 to 3 Ma). Here we present new multi-proxy, multi-site paleotemperature records from the WPWP. These results, based on lipid biomarkers and foraminiferal Mg/Ca, unequivocally show warmer temperatures in the past, and a secular cooling over the last 10 Myr. We combine these new data, along with the previously established paleotemperature records, to reveal a persistent pattern of change in the Pacific described by a high latitude amplification factor of ~1.7, which does not seem to be affected by the major climate changes over the past 10 Myr. The evolution of spatial temperature gradients in the Pacific is also evident in climate model output and instrumental observations covering the last 160 years, and thus appears to be a robust and predictable feature of the climate system. These results therefore confirm that climate models can capture the major features of past climate change, providing increased confidence in their predictions of future patterns that are likely to be similar to those reconstructed here.

How to cite: Liu, X., Huber, M., Foster, G. L., Leckie, R. M., and Zhang, Y. G.: Persistent high latitude amplification over the past 10 million years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-597, https://doi.org/10.5194/egusphere-egu2020-597, 2020.

The morphological evolution was investigated in the tropical Neogene planktonic foraminiferal lineage Globorotalia menardii, G. limbata and G. multicamerata during the past 8 million years at ODP Hole 806C (Ontong-Java Plateau). This research is an extension of previous studies from the Caribbean Sea, the tropical Atlantic and the Eastern Equatorial Pacific.

The peripheral influence of Agulhas Current faunal leakage of Indian Ocean or even Pacific menardiforms into the South Atlantic is suspected to be responsible for a transgressive, transatlantic expansion of large menardiforms from 2.3-2.06 Ma to 2.58-1.7 Ma, which installed after a Northern Hemisphere Glaciation (NHG) size incursion of menardiforms around 2.6 Ma (Knappertsbusch, 2007 and 2016; Knappertsbusch & Friesenhagen 2018). The investigation from Western Pacific Warm Pool (WPWP) ODP Hole 806C, i.e. from an area outside reach of Agulhas Current, serves as a blind test for this szenario. Here, stable warm environments prevailed back to Pliocene times, and influences of NHG are expected to bear less severely on shell size evolution than in the Atlantic Ocean.

For this study >5250 specimens comprising G. menardii, G. limbata and G. multicamerata from 33 stratigraphic levels were morphometrically investigated using imaging- and microfossil orientation robot AMOR. Attention was given to trends of spiral height (δX) versus axial length (δY) in keel view, for which bivariate contour- and volume density diagrams were constructed for visualization of evolutionary patterns.

In WPWP Hole 806C G. menardii evolved in a more gradual manner than in the Atlantic. Plots of δX versus δY reveal bimodality between 3.18 Ma – 2.55 Ma with a dominant mode of smaller G. menardii (δX<~300 μm) persisting until the Late Quaternary, and a weak mode of larger G. menardii (δX>~300 μm) until 2.63 Ma. Up-section, bimodality vanished but G. menardii populations shifted towards extra large shells between 2.19-1.95 Ma supporting the possibility of long-distance diversal in this group. Morphological evolution of G. limbata and its evolutionary successor G. multicamerata in the WPWP are also different from those in the tropical Atlantic, but analyses need still further investigation.

In summary, Pacific menardiform globorotalid patterns contrast those in the Atlantic realm. There is inter-oceanic morphological asymmetry with considerable regional environmental control over shell evolution and indication of long-distance dispersal of G. menardii, both with implications for biostratigraphic applications.

References

Knappertsbusch, M. and Friesenhagen, T. (2018). Prospecting patterns of morphological evolution in menardiform globorotalids along Agulhas‘ trackway: Review and research in progress. Abstract. FORAMS 2018 Symposium, 17-22 June 2018, Edinburgh, UK., Session IX, temporary abstracts, 331.

Knappertsbusch, M. (2016). Evolutionary prospection in the Neogene planktic foraminifer Globorotalia menardii and related forms from ODP Hole 925B (Ceara Rise, western tropical Atlantic): evidence for gradual evolution superimposed by long distance dispersal ? Swiss Journal of Palaeontology, 135, 205-248.

Knappertsbusch, M. (2007). Morphological variability of Globorotalia menardii (planktonic foraminifera) in two DSDP cores from the Caribbean Sea and the Eastern Equatorial Pacific. Carnets de Géologie / Notebooks on Geology, Brest, Article 2007/04. http://paleopolis.rediris.es/cg/CG2007_A04/index.html.

More info: https://micropal-basel.unibas.ch/

How to cite: Knappertsbusch, M.: Morphological evolution of menardiform globorotalids at ODP Hole 806C (Ontong-Java Plateau), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2396, https://doi.org/10.5194/egusphere-egu2020-2396, 2020.

With the final closure of the Tethys Seaway during the early Miocene the Indian Ocean came into existence as geographic entity. The total breakdown of faunistic interrelations between the proto-Mediterranean Sea and Indian Ocean during the Aquitanian suggests a major biogeographic separation around ~22 Ma somewhere between Mesopotamia, Arabia, and NW-India. This event predates the development of the so-called Gomphotherium landbridge in the Burdigalian by 4-5 Ma and provides an example for biogeographic separation in the marine realm without formation of a continental barrier. The benthic mollusk fauna of the Miocene Indian Ocean was far from uniform. Data on Oligocene and Miocene mollusks from southern Iran (Qom Basin, Makran), the Sultanate of Oman, Tanzania, northern India (Kutch), southern India (Kerala) and Sri Lanka document a complex pattern of faunistic relations between these areas with high rates of endemism. Thus, a strong early Miocene bioprovincialism can be postulated for the Proto-Indo-West Pacific Region with a Central East African Province, a East African-Arabian Province, a Western Indian Province and a Proto-Indo-Polynesian Province in the east. This pattern differs fundamentally from the modern biogeography in the Indian Ocean. Similarly, fossil reef coral faunas from Eastern Africa (Tanzania, Somalia) show a low relation with southern Iran and no relationship with Indonesia during the Oligocene and early Miocene, but a strong faunistic affinity with Indonesia during the late Miocene and Recent.

We postulate that three main factors explain the strong difference between the Oligocene-Miocene and modern biogeographic patterns: the absence of the Miocene Indian Ocean Equatorial Jet, which did not arise before the middle Miocene, the absence of the Indonesian Throughflow due to the completely different paleogeography, and finally the devasting effect of Pleistocene sea-level fluctuations and subsequent recolonizations of the Indian Ocean from the east.

This project was funded by the Austrian Science Fund (FWF, Grants P 18189-N10 and P 29158-

N29).

How to cite: Harzhauser, M., Reuter, M., and Piller, W. E.: Oligocene-Miocene benthos biogeography of the Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5309, https://doi.org/10.5194/egusphere-egu2020-5309, 2020.

During IODP Expedition 363, a hemipelagic sediment succession was retrieved for the first time off NW Australia (Site U1483: 13°5.24ʹS, 121°48.25ʹE, water depth: 1733 m, sedimentation rate: ~10 cm/kyr). This carbonate- and clay-rich sequence provides an ideal archive to monitor intensity and variability of the Australian Monsoon (AM) and to better constrain monsoon sensitivity to changes in radiative forcing. Due to the location at the southern edge of the largest amplitude seasonal swing of the Intertropical Convergence Zone (ITCZ) within the large-scale Asian-Australian monsoon system, the AM subsystem is sensitive to tropical hydroclimate variability. However, this sensitivity to changing climate boundary conditions such as ice volume and greenhouse gas concentrations remain poorly understood across the Calabrian, in the Pleistocene Epoch (add ages?). Here we report on benthic foraminiferal assemblages as a tracer for terrigenous runoff (Austral Summer monsoon precipitation). We find shallow, fresh water-tolerant to transitional environments species Bolivina striatula, Buliminella elegantissima, Dentalina spp., Oolina sp., and paleo productivity indicator Melonis spp (winter monsoon wind-driven upwelling) from 1.34 Ma. through 1.61Ma. Principl component analysis (PCA) indicates that Melonis is present in 4 out of 5 PCA axes. It prefers organic matter in a more altered form, and migrates in the sediment depending on the quality of the organic matter supply and remineralization, which indicates surface upwelling during this time. The genera Stilostomella spp. is present in 3 out of 5 axes, and it is indicative of intermediate water temperature. These records will be compared to C org wt (%), TN wt (%) and a benthic foraminiferal stable isotope record to related faunal patterns to carbon cycling and global climate.

How to cite: Eichler, P., Billups, K., Ravelo, A. C., and Vital, H.: Benthic Foraminifera as indicative of Austral Summer monsoon precipitation and winter monsoon wind-driven upwelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8387, https://doi.org/10.5194/egusphere-egu2020-8387, 2020.

The tropical Pacific thermocline structure is critical to tropical sea surface temperatures (SSTs) and variability. During the mid-Pliocene warm period (~3 Ma), the zonal SST gradient was reduced due to relatively warm SST in the Eastern Equatorial Pacific; we call this mean state “El Padre.” How did the equatorial thermocline contribute to this reduced zonal SST gradient? Here we summarize published Mg/Ca (surface and subsurface dwelling foraminifera) and alkenone records and generate new SST estimates from Mg/Ca and alkenones. The subsurface dwelling Globorotalia tumida Mg/Ca-based temperature records from the eastern and western equatorial Pacific show mid-Pliocene warm period subsurface temperatures warmer than today; El Padre included a basin-wide thermocline that was relatively warm, deep, and weakly tilted. We compare the published and newly generated SST and subsurface temperature records to the Pliocene Modeling Intercomparison Project (PlioMIP1) and show that few models capture the magnitude and spatial pattern suggested by the temperature records. Those models that do corroborate the temperature records have warm subsurface temperatures in the Eastern Equatorial Pacific that dynamically link to warm SSTs in the cold tongue. This highlights the need to accurately model thermocline dynamics and mid-latitude conditions, where equatorial thermocline waters originate, in order to gain an understanding of the underlying processes that explain the mid-Pliocene warm period.

How to cite: Ford, H. L., Burls, N., Chandan, D., LaRiviere, J., Fedorov, A., and Ravelo, A. C.: Beyond Sea Surface Temperatures: a Holistic Approach to Addressing Pliocene Tropical Conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9060, https://doi.org/10.5194/egusphere-egu2020-9060, 2020.

Presently, the Indian Ocean exhibits a unique climate state with subtle east-west contrasts and weak year-to-year variability. Whether these features could change in response to external forcings remains highly debated, an issue that is critical to predict future climate changes in highly populated neighboring countries. We explored this question combining climate reconstructions and numerical simulations and of the Last Glacial Maximum – the interval ca. 21,000 years ago when the Earth experienced the largest, most recent climate change. We found that the Indian Ocean exhibited radically altered rainfall patterns and oceanographic conditions across the basin, changes that according to our simulations, can only be explained by the amplifying effect of coupled ocean-atmosphere feedbacks. We also find that these changes favored the emergence of an El Niño mode driving significantly stronger climate variability. Despite different triggers in the past and the future, our results show that Indian Ocean climate could also be highly sensitive to future greenhouse forcing.

How to cite: DiNezio, P.: Past and future changes in tropical climate amplified by the Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11494, https://doi.org/10.5194/egusphere-egu2020-11494, 2020.

The recent documentation of the southern hemisphere “supergyre”, the coupled subtropical southern hemisphere gyres spanning the 3 ocean basins, leads to questions about its impact on Indian Ocean circulation. The Indonesian Throughflow (ITF) acts as a switchboard directing warm surface waters towards the Agulhas Current (AC) and return flow to the North Atlantic, but Tasman Leakage (TL) is another source of return flow, however, at intermediate water depths. Fed by a complex mixture of South Pacific (SP) western boundary current surface and intermediate waters, and Antarctic Intermediate Water (AAIW), today the topography forces it to flow in a westerly direction. The TL flows over the Broken Ridge towards Madagascar, joining the AC and ultimately Atlantic Meridional Circulation (AMOC).

Stable isotope data from 4 DSPD/ ODP Indian Ocean sites define the history of TL and constrain the timing of its onset to ~7 Ma.  A simple nannofossil- biostratigraphy age model applied to previously published benthic foraminiferal carbon isotope data ensures the 4 time-series (~11 – 2 Ma) are consistent. All 4 records (Sites 752 Broken Ridge, 590 Tasman Sea, 757 90 East Ridge, 751 Kerguelen Plateau) are similar from ~11 Ma to ~7 Ma, indicating the Tasman Sea intermediate water was sourced from the Southern Ocean (SO). A coeval shift at ~7 Ma at Sites 590 and 752 signals a SP contribution and the onset of TL. We do not observe TL at Sites 757 and 751 and so interpret the post-7 Ma divergence between the TL pair and the KP / 90E Ridge sites as a reflection of different intermediate water masses. The KP / 90E Ridge sites record a more fully SO signal, and these waters are constrained to the region west of the 90 East ridge.

The isotopic record of TL onset suggests important tectonic changes ~ 7 Ma: 1) opening of the Tasman Sea to the north and 2) Australia’s northward motion allowing westward flow around Tasmania. The former is supported by a change in sedimentation style on the Marion Plateau (ODP Site 1197). The latter is supported by unconformities on the South Australian Bight margin (Leg 182 Sites 1126 (784 m), 1134 (701 m), 1130 (488m) and coeval decreases in mud- sized sediments at the Broken Ridge sites, indicating winnowing associated with the onset of the TL. A divergence is also apparent between Broken Ridge and Mascarene Plateau Site 707 records at this time. These events, coupled with the temporal relationship between the onset of the TL and a change in the character of deposition in the Maldives indicate enhanced Indian Ocean circulation at intermediate depths coincident with the late Miocene global cooling. Combined, these observations suggest the Indian Ocean in general plays a larger role in the global ocean system than previously recognized, and intermediate waters in particular are a critical yet poorly understood component of AMOC.

How to cite: Christensen, B., DeVleeschouwer, D., Groeneveld, J., Henderiks, J., Auer, G., Beztler, C., Eberli, G., Drury, A. J., and Kroon, D.: Geochemical and Geophysical Evidence for Late Miocene Onset of Tasman Leakage, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11701, https://doi.org/10.5194/egusphere-egu2020-11701, 2020.

The Western Pacific Warm Pool (WPWP) as a major source of heat and water vapor has a crucial influence on climate dynamics both in the tropics and globally. Yet, there is conflicting proxy evidence regarding the evolution of WPWP temperatures since the Miocene. On the one hand TEX₈₆ data suggest a gradual cooling by ~2℃ (O’Brian et al., 2014, Zhang et al., 2014) from the Pliocene to today, while faunal (planktonic foraminifera) sea surface temperature estimates (Dowsett, 2007) and Mg/Ca data measured in planktonic foraminifera (Wara et al., 2005) on the other hand indicate the absence of any long-term temperature trends. It has been suggested that Mg/Ca temperatures could on these time scales be biased by long-term changes of the Mg/Ca ratio of seawater (Evans et al., 2016). To test the influence of the proposed seawater changes on Mg/Ca we combined data from two independent temperature proxies, Mg/Ca and clumped isotopes, measured on two species of planktonic foraminifera from IODP Site U1488 in the central WPWP. Our study finds good agreement between both proxies thereby verifying the validity of Mg/Ca records from the WPWP and confirming the absence of a Plio-Pleistocene cooling trend for the WPWP. This finding suggests that the persistent disagreement between foraminifer-based proxies such as Mg/Ca and biomarker data might be caused by different environmental parameters being recorded in the two archives.

References:

O’Brien CL, Foster GL, Martínez-Botí MA, Abell R, Rae JWB, Pancost RD. High sea surface temperatures in tropical warm pools during the Pliocene. Nature Geoscience. 2014;7(8):606-11.

Zhang YG, Pagani M, Liu Z. A 12-million-year temperature history of the tropical Pacific Ocean. Science. 2014;344(6179):84-7.

Dowsett H. Faunal re-evaluation of Mid-Pliocene conditions in the western equatorial Pacific. Micropaleontology. 2007;53(6):447-56.

Wara MW, Ravelo AC, Delaney ML. Permanent El Nino-like conditions during the Pliocene warm period. Science. 2005;309(5735):758-61.

Evans D, Brierley C, Raymo ME, Erez J, Müller W. Planktic foraminifera shell chemistry response to seawater chemistry: Pliocene–Pleistocene seawater Mg/Ca, temperature and sea level change. Earth and Planetary Science Letters. 2016;438:139-48.

How to cite: Meinicke, N., Reimi, M., Ravelo, C., and Meckler, N.: Coupled Mg/Ca and clumped isotope measurements at IODP Site U1488 confirm absence of Plio-Pleistocene sea surface temperature cooling in the Western Pacific Warm Pool, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16364, https://doi.org/10.5194/egusphere-egu2020-16364, 2020.

As the largest modern reservoir of oceanic heat, the Western Pacific Warm Pool (WPWP) plays an important role in atmospheric and oceanic circulation patterns. Little is known about how regional deposition patterns have changed over the past 10 Ma. To understand the interplay between regional processes and global climate evolution in the WPWP, we explore the late Neogene evolution of biogenic (carbonate/siliceous) versus terrigenous deposition.

We collected high-resolution (2 cm/~0.5 kyr) X-Ray fluorescence (XRF) core scanning data at IODP Site U1488 (Exp. 363) in the central WPWP. These data were especially useful for estimating the carbonate, siliceous and terrigenous components below 65 m CCSF, where the shipboard track data were less robust. The shipboard splice was verified and revised using the Ba/Sr ratio to ensure a continuous composite section down to ~330 m revised CCSF-A at Site U1488. Fe and Si likely reflect terrigenous and partially biogenic silica components. We calibrated the high-resolution ln(Ca/K) record to %CaCO₃ using discrete shipboard %CaCO₃ measurements.

Fe and Si decrease, whilst ln(Ca/K) increases downcore, in agreement with shipboard data showing increasing %CaCO₃ and decreasing terrigenous/siliceous input­. During the late Pleistocene, the site shows high amplitude %CaCO₃, Fe and Si cycles superimposed on low carbonate. The amplitude decreases during the early Pleistocene-mid Pliocene, although clear variability remains. The early Pliocene-late Miocene is dominated by high CaCO₃ (80-90%). The %CaCO₃, Fe and Si variability is considerably reduced, although clear obliquity-precession interference patterns are visible, in addition to longer-term ~400 kyr eccentricity modulation. The high-carbonate interval at IODP Site U1488 likely reflects the early Pliocene to late Miocene Biogenic Bloom (LMBB). The expression of the LMBB in the WPWP is distinctly different to the Atlantic and eastern equatorial Pacific. This indicates that although productivity was enhanced during the late Miocene-early Pliocene, regional processes determined the exact expression and timing of the LMBB in different areas.

How to cite: Drury, A. J., Westerhold, T., Ravelo, A. C., Aiello, I., Wilkens, R., Röhl, U., and Kulhanek, D.: Late Neogene carbonate productivity and terrigenous input in the central Western Pacific Warm Pool (IODP Site U1488), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17079, https://doi.org/10.5194/egusphere-egu2020-17079, 2020.

The coupled ocean-atmosphere circulation of the Indian Ocean Dipole (IOD) controls monsoon rainfall in eastern Africa and southeast Asia at seasonal to decadal time-scale. In years when the dipole is particularly active, it can lead to catastrophic floods and droughts. A growing body of evidence suggests that IOD variability influenced the continental hydroclimate also at longer timescales in the past and thus may have affected human evolution.  However, long-term continuous high-resolution well-dated records have so far been unavailable to test this hypothesis. In 2016, long-term continuous deep-sea sediment cores have been recovered from the Davie Ridge in the Mozambique Channel during Expedition 361 ‘Southern African Climates’ as part of the International Ocean Discovery Program (IODP).

Here, we present a more than seven million-year-long multi-proxy record of Mozambique Channel Throughflow (MCT), which is tightly coupled to IOD variability; defined here as the zonal sea surface temperature gradient (ΔSST) between the Indo-Pacific warm pool (IPWP) and the Arabian Sea. We show that the MCT was relatively weak and steady until 2.1 million years ago (Ma), when it started to significantly accelerate with progressively increasing glacial-interglacial amplitude, culminating in high flow speeds from 0.8 Ma onwards. The invigoration of MCT activity coincided with increasing zonal ΔSST, which fuels the atmospheric Walker Cell circulation along the tropical Indian Ocean.  Our results demonstrate that the overall intensification of the Indian Ocean Walker Cell amplified the coupled ocean-atmosphere Indian Ocean zonal circulation at orbital time-scales, which agrees with the heightened glacial continental aridity recorded in other eastern African climate proxy records. We argue that the corresponding progressively drier glacials alternated with relative humid interglacials, providing the climatic-environmental setting –varying at seasonal to orbital timescales- for speciation and global expansion of our genus Homo after 2.1 Ma.

How to cite: van der Lubbe, J., Hall, I., Barker, S., Hemming, S., Just, J., Baars, T., Joordens, J., Starr, A., and 361 Scientists, E.: Invigoration of Indian Ocean zonal circulation drove Pleistocene eastern African aridification , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19624, https://doi.org/10.5194/egusphere-egu2020-19624, 2020.